My answer has more to do with the groups that the Tlingit and other Northern Coast groups would get into conflict with on the Southern Coast (Southern BC into WA), particularly Coast Salishan groups.

Both of these make me think that war was mostly slave raids by the elite of the region against other peoples of the Pacific Northwest and neighboring regions and their peoples (this is a complete guess).

A fairly astute guess, slaves do need to come from somewhere and as such, warfare often involved raiding for slaves.

who were the warriors, why did they fight, was it rare or constant?

I'll break this down within the contexts of Coast Salishan groups as I know them best and can characterize their methods of warfare easier.

#1: Who were the warriors?

For the eponymous peoples of the Salish Sea, from Sto:lo to Nisqually, there is a consistent understanding of what warriors are (though their criteria varies from North to South): A professional warrior differs from a random tribesman who is picking up a spear to defend his village. There is no illusion that what a professional warrior does is remotely noble or honorable unless they're in the middle of defending the tribal group from a current threat.

This being said and I noted, the understanding of just who or what constitutes a professional warrior varies a little from Northern to Southern Coast Salish. While the Northern, Central, and Southern Coast Salish could agree that the Warrior was a figure to be avoided, David E. Jones, in "Native North American Armor, Shields, and Fortifications" (which I strongly recommend) sums up the general understanding like so:

Professional warriors, also found among the Coast Salish tribes, delighted in causing fear and consternation among their fellow villagers. The southern Salish tribes believed terrifying spirits, which they had been trained to seek in dreams and vision-seeking expeditions, animated these "mean men." When going into battle, such men, through songs taught to them by their war spirits, could whip themselves into a berserk frenzy. Among northern Salish tribes, warriors did not seek spiritual powers to make them wild in battle but rather considered the role as a professional position which ran in families. Fathers in family lines that did not have the prerogative to create professional warriors still sought to elicit the desire to fight well in at least one of their sons.

Though I cannot comment on the latter summation given about Northern Coast Salishan groups, I will note that he seems to be interpreting Southern Coast Salishan power songs as taking place immediately before a battle. I haven't seen any other references to that specific claim though power songs were undoubtedly part of a Warrior's lifestyle, I think Jones might be making it more metal than it would have really been (but it is a cool visual, right?).

An observation that best sums up their interpretation of Warriors is best given by Florence James, a Penekalut Elder who relayed her understanding of the Cowichan Warrior Tzouhalem* thusly:

"Everyone knows him as a bad man...and as a man like a monster. But, to me he was a warrior" (Angelbeck and McLay 2009)

As such, the consistent view of Coast Salishan Professional Warriors we get is that they (even if they are part of the nobility and come from prestigious families) are often on the outskirts of village life due to their temperament and line of work.

#2: Why did they fight?

Slave Raids:

As you noted, warfare could be undertaken for procuring slaves. However, with the emphasis given to marrying outside the tribal community (preferably to tribes that the family has few or no ties with), slave raids were taken against communities distant enough to where they could reasonably assume no one in the war party had any blood ties to. As a result of seeking out unrelated and distant groups to attack, it has been noted by Jay Miller and others that one could have feasibly (and has) been captured from Vancouver Island and be sold to tribes along the Columbia River between Washington and Oregon.

Disagreements:

Probably the most common reason for war among Coast Salishan groups, though we should keep in mind that this can range from simple skirmishes over a brief period of time to full blown sieges on fortified villages.

Gibbs notes a the presence of formal and prearranged battles by local tribal groups. His account notes that prior to the exchange of blows over the waters of the Sound, that there were attempts to come to an understanding and agreement by representatives of both tribes (presumably chieftains or other persons known for their skill with words) in order to avail bloodshed. If this failed, then a battle commenced via canoe wherein combatants exchanged spear blows while leaning the sides of their canoes toward the other. Notably, this affair seemed to be rarely deadly as Gibbs notes "Owing to the curve of their canoes, and their impenetrable armor, it is seldom bloody... The same description will apply to most of the battles on the Sound except where northern tribes are concerned, who are more warlike and ferocious. (Gibbs 1877, 190-191)".

Tribal Affiliation (?):

Unusually, there are also a few accounts of a particular Puyallup Warrior referred to as "Zachias" in Marian Smith's 1940 work "Puyallup-Nisqually". Now the reason he fought seems unusual since while many Warriors fought for a variety of reasons, ranging from petty grievances to mourning to their power invoking a lust for blood, they tend to respond to direct threats affecting them or their relatives as opposed to the wider tribal group...except for Zachias who is consistently noted to be fervently against letting any Puyallup being taken as a slave.

(2) Once some Puyallup were all down at the beach to get clams. The men were off somewhere when two strange men landed. The women fed them and when they finished, the men made one of the young women and her children go in the canoe with them. They were taking them and when Zachias got back and found out what had happened, he was mad. The others didn't want to do anything. But Zachias made one of the men go in his canoe with him and started in pursuit. He drew alongside the other canoe and told the woman and her children to get in with them told the men that he would let them go but, if he ever caught them a second time, he'd kill them. When he brought the woman back her husband had returned. Her husband said that he would have killed the men and given them another chance. Zachias never let a Puyallup be taken a slave.

(3) Once Zachias was away and when he got back he found out that some Indians had come in a canoe and stolen a girl. He made the men with him after the girl. When the other Indians saw the Puyallup coming they stripped the girl and left her on the beach. Zachias got her brought her back. When she was grown her parents gave her to Zachias as a second wife. There were no Puyallup slaves anywhere, he wouldn't allow it.

(4) A man came here once asking for things and all the people cowered down and gave them to him. He was going to take Sitwell's (Sitwell was village leader) son away as slave. But Zachias said, "No." He jumped up with his knife and threatened the man. He went away then. Zachias never let anyone of Puyallup get taken slave.

---Smith, 1940, pp.160

To emphasize why this is peculiar, it's that tribal identity tends to be widely seen as a more recent innovation with the establishment of (though of course, this does have its detractors even in the earlier days of anthropological study within the Puget Sound^). To have Tribal Elders recount a figure that is not only directly as the progenitor of a still present Puyallup family, but that those giving these accounts were only a generation or two away from him is remarkable.

Outside Threats:

As the excerpt from Gibbs notes, the Northern groups who automatically come to mind when "Northwest Coast" is brought up had little interest in partaking in a formal exchange of blows with honor satisfied. With that, this also meant that they had no allies within the region (outside of potentially more Northern Salishan groups with the Kwakwakaʼwakw) who would come to their aid or had any real initiative to not attack these foreigners. This resulted in efforts at organized responses, such as an example I've provided below describing the effects that Euro-American contact had, with Salishan groups understanding that they have a shared threat.

#3 4: How did Euro-American contact change war in the Pacific Northwest?

The introduction of firearms among Northern groups led to increased raids on Southern ones. This was particularly prominent with the Lekwiltok of the Campbell River area of Vancouver Island, eventually culminating in an alliance of Salishan tribes from British Columbia to the Southern Puget Sound being gathered to meet them at Maple Bay in 1840 and prevent them from being a further threat.

As a result of the introduction of firearms, armor faded from use alongside the more traditional implements of warfare such as spears, clubs, certain types of daggers, and bows. However, while they became less common, they were still present in the Puget Sound Indian War of 1855-56 with the occasional adoption of Euro-American tools and other implements for battle.

Sources Used:

• "The Indians of Puget Sound", Haeberlin and Gunther, 1921

• "Puyallup-Nisqually", Marian Smith, 1940

• "Native North American Armor, Shields, and Fortifications", David E. Jones, 2004

• "Lushootseed Culture and the Shamanic Odyssey", Jay Miller, 1999

• "They Recognize No Superior Chief", William O. Angelbeck, 2009

*if anybody who is familiar with the Cowichan language knows how I would put that in Cowichan as opposed to keeping it anglicized, I would appreciate the opportunity to properly amend it as I do with Lushootseed names.