I'm going to try to address your points in order. Firstly I'll state that Scandinavia had been a state of conflict for some time. To get a sense of the politics, you may sit down one day and read Njal's Saga, as Palle Rassmussen would suggest. Magnates did as they chose to, while strong and charismatic kings were able to keep them in check or use them. In 1286, Eric V of Denmark was murdered, which would be the source of a lot of conflicts within Denmark internally, the dominating power and therefor influenced it's neighbors in various degrees leading up to Nyköpling Banquet. Norway focus more time on Scandinavia ever since the Treaty of Perth. Sweden was a place with little central power, but strong dukes and free farmers.

Although the killing gave the northern conflicts a new side to warfare, one that reshaped the political landscape drastically, by the fact that the 3 most powerful princes omitted from their throne, the debate was often heavily leaning towards supporting your alliances rather than a simple unified moral stance. We see this in wars even today, with people justifying their own point of view, rather than see their own side as evil. This meant that outside realms who supported the King of Sweden, painted the conflict quite differently. According to Ingvor, Chronica Jutensis, Detmar "Lübercker" Chronicle and Annals of Lübeck all focused on the treatment of Magnus Birgerson, the heir to King Birger of Sweden. These were areas under the influence of the Danish King Menved, the in-law of the Swedish king, who all painted the treacherous act on the Swedish royal heir in their view. That the heir was executed, despite the fact that he was crowned, and regardless of the fact that Swedish nobility had swore allegiance to him, but had changed side. Andersson, Ingvor M., Erik Menved och Venden: Studier i dansk utrikespolitik 1300-1319, Lund 1954.

Duke Eric's father in-law, King Haakon of Norway wasn't mentioned in regards to these events in the Eric Chronicles, but we know from the Icelandic annals of a few events that transpired from Norway as a response. During this period, the Norwegian King resided safe and sound in his stronghold in Tunsberg. Here he could launch his fleet either to support the forces of the two Dukes or against King Erik Menved of Denmark. In 1318, the Norwegians had a fleet in River of the Geats to assist the forces of the Swedish Dukes. One of the ships supposedly got frozen and stuck. At this point King Birger's men attacked. The Norwegians surrendered, but only before their heads were chopped clean off. Islandske Annaler, 394.

The context of Scandinavia was a lot more fluid than it is today. Lords (mostly dukes and other magnates) acted on their own behalf, often changing allegiances between princes were they saw fit and according to their social network. We can however draw some distinctions. A German Noble in Scandinavia would not find much support from anyone other than his Scandinavian prince, which meant they were often more loyal than native Scandinavians troops, who could switch sides. This was because Scandinavian lord's social network were interwoven and often had affiliates in both camps of any conflict. One Scandinavian example is Tore Unge, a Norwegian commander who constantly changed sides during the 1309-10 armed conflict between the Norwegian King and Duke Eric of Sweden, depending on who was knocking on his castle, which was located in the border area. A contrasting example is the German adviser Johan Brunkow, son of Werner Brunkow (who was one of the most loyal men to Magnus III of Sweden). While none of the Swedish magnates followed King Birger of Sweden when he retreated to Gotland, the German adviser remained by his side til the end of his dishonoured execution. The same example is something we see in the massive 1309 campaign into Sweden by King Erik Menved of Denmark. According to Chronica Jutensis, the Danish part of the army lead a mutiny, while the German mercenaries remained loyal to the King, preventing a bigger disaster to enfold in the heart of the enemy land. So to answer your question, the Swedish nobility supported who was strong. King Birger of Sweden was anything but that. As a general rule for Swedish Medieval kings, to quote Raymond Johansen's Ma, The Political Impact of Crusader Ideology in Sweden 1150 - 1350

"... the absolute power of the (Swedish) king did not exist even in theory and his influence largely depended on the support of his peers..."; "power is one thing, legitimacy another. Kings had come and gone, usually in violent ways such as with King Eric XI, telling us that the legitimacy on which their power rested was constantly held in doubt, challenged and often undermined. The position of the kings was therefore anything but unassailable, on the contrary it seems to have been far too fragile for them to risk unpopularity by forcing reforms on society on the same scale as Birger (Jarl) did."

This was the case for the Swedish King. What basically happened was war. The weak King first failed when attacking Stockholm, the Dukes eastern fief. In an attempt to recover, he further submitted to the Danish king in February 27, 1318, and in return obtained mercenaries and backing from King Menved of Denmark. The supporters of the two Dukes besieged Nyköping Castle (one of the strongest in the realm), while the Swedish King tried to cut their supply routs. He obtain brief lead after winning a battle, before he lost a battle later. And with that, the Danish mercenaries returned to their homes.

The Eric Chronicles characterized the events of Nyköpling Banquet in favor of Duke Eric side of the story, relating the Banquet to Biblical stories. Comparing King Birger to Judas, while Duke Eric to Jesus, and much more, which shouldn't be underestimated at a time of deep religiosity in Europe at the time. The situation of the Siege of Nyköping Castle eventually became desperate for the defenders. Eventually, the castle of Nyköping was destroyed completely, "not one stone on another". Once again Eric Chronicles allude to biblical references, "not one stone here will be left on another", Matt 24.2 The fact that powerful castles (Nyköpling, and later Stegeborg) of King Birger were destroyed gives us some idea behind the revenge that followers of the Dukes wanted.

The church usually intervened when one of their own were targeted. There were a few cases were the Pope would put heavy pressure on the Scandinavian monarchs, but no such case in this specific scenario that I'm aware of. The only instant I'm aware of is when King Birger of Sweden erected new taxes for the Archbishop of Uppsala, one of his strongest supporters (even when this Tith of Tiundland, a fief of Duke Valdemar of Sweden, had already been payed 3 years in advance). However, I'm not aware of the church being involved as a major actor beyond that.

King Birger was, you may say, a pawn of his in-law, King Menved of Denmark. He started out as one of the weaker princes in Sweden leading up to the event. In the final treaty of Roskilde, the King of Sweden wasn't even physically present. He had functionally no political power at that point what so ever, other than his title. When we see the economic borrowing from King Menved to Birger, we see a King with little authority. He was too reliant on the Danish king, he had German advisers, he had mercenaries from Denmark and Danish Germany, he pawned land (Småland, Visingö) to the Danish King and erected castles there. In addition, he gave away supreme decision making in military and political matters to the danish king in February 27, 1318 in return for those 600 mercenaries as well as further waning support. The final treaty was mainly between the followers of the Swedish Dukes and King Menved of Denmark. It's therefor possible to assume King Birger of Sweden didn't stage a risky coupe on his own. Tor E. Fagerland: Krigføring og politisk kultur i nordisk middelalder. 2005.

Letters of debt to Erik Menved of Denmark:
1310: 10 000 Mark silver
1311: 4000 Mark silver
1312: 2000 Mark silver
1313: 650 Mark silver
1314: 1500 Mark silver coins to Erik Menved's ally, Henry of Mecklenburg.

I'm sorry for any spelling mistakes, but I hope this helps for now. :) Cheers.

Also I have heard that Erik and Valdemars testamen should still exist, but I can't seem to find them. Does anyone know where to find them?