Generally speaking, asking "was WWI inevitable" is a slightly vague and inadequate question. I don't say this to be disparaging, but to answer "inevitability" questions you have to start looking at the causes and of course an event as huge as the First World War has a plethora of causes that happened over many years, even decades.

For example, you could say that one of the cause of the First World War was the fact that Germany (or its predecessors, Prussia etc) were incensed by being invaded several times by France in the 19th century and wanted to put a stop to it. Does this mean that by 1871 it was inevitable that there would be another war between Germany and France that escalates into a World War? No probably not, at this point it probably wasn't inevitable. What about industrialisation and the fact that this gave rise to the new mode of war that was seen in WWI, did industrialisation mean that some World War would occur because war always happens somewhere and this would simply be the first time industrialised nations understood their new technological capabilities? Well, perhaps industrialisation alone did make a world war inevitable. But people at the time certainly wouldn't have thought so.

What then about the race for Africa - imperialism escalating throughout Europe but played out on other continents. Well this probably is the point where folk of the time were thinking that everything would probably come to a head in a large scale war. This is evident by the way alliances started forming for various potential eventualities. The arms race would have made it a near certainty and the governments of the time would have known it by this point. Tensions were so high that people were looking for an excuse to pull the trigger and launch a war. Franz Ferdinand's assassination definitely wasn't THE cause, but it is widely recognised as that trigger, that excuse that Germany used to launch the Schlieffen plan and invade France via Belgium.

So perhaps a better question is, "at what point did large scale war become inevitable/unavoidable". Of course we can't pick a date but I have put across a couple of ideas above.

The second part of your question is interesting. What we find in 20th century war is Britain and France on the same side for the first time in 1000 years (ish). You could point to that and say that not many people would have predicted a few decades earlier the staunch friends these two countries would become (politically). You might therefore think it "natural" for Britain to side against France - but that was never on the cards in the early 20th century because the perceive threat was coming from Germany and the arms race. Alliances were needed against them so Britain and France threw in together. That and recent diplomatic ties over how to divide Africa - Britain and France were working together here in order to avoid costly wars.

But we should also discuss here the fact that Russia switched sides. Russia was allied in the 1870's with the German and Austro-Hungarian side, but when relations between Russia and Germany thawed substantially, their alliance was not renewed, Russia panicked and threw in with the French as well.

So depending on how far back you look I would suggest that no, the alliance systems (which BTW were key to building the paranoia that helped lead to war in the first place) were not foregone conclusions in general, they could have gone in any direction depending on how various events unfolded over the decades preceding the war itself.

Not so sure about the US on that point though. I find it unlikely they would have gone against Britain and France in any situation for historical reasons. Historically, culturally, and economically they were interlinked with both countries and if they were ever going to come into the war (and they took their sweet time about it), it was going to be on the side of the Triple Entente.

I won't go into the issue of inevitability as that's a rather broad issue, but the Alliance system is possibly more easy to deconstruct. Whilst the gist of the sides were broadly as you described, they were certainly not cemented the outbreak of the war. Italy was originally in an alliance with Germany and Austria-Hungary, but as this was only a defensive alliance they did not feel they had to intervene (plus as part of the irredenta they thought they might be able to get land concessions from the Allies). If Austria had offered something along these lines, the Italians might have joined the Central Powers.

The Ottoman Empire, whilst having strong trade links with Germany was incredibly backwards (militarily/industrially), and entered the war reluctantly only after much cajoling from the Germans (remaining neutral for the first 6 months of the war), and so it is not unfeasible that they might not have entered the war at all.

Whilst the UK had improved relations with France and Russia, I don't think any clauses in the Triple Entente actually obliged the UK to intervene if either nation were attacked. It is cited that the UK only intervened in response to the invasion of Neutral Belgium, with whom they had an explicit alliance since the 1830s. Whilst it is unlikely that the UK would have tolerated German expansionism if they hadn't violated Belgium's neutrality, it is a possibility to consider.

The issue of American neutrality is more complex with internal factions both in favour and against intervention, but German aggression largely forced their hand regarding unrestricted warfare against civilian shipping/ the Zimmerman Telegram.

So basically, whilst it was likely to guess who would intervene on which side, events could have changed the line-up as it were

https://www.youtube.com/watch?v=Ay22kuic4JA

this documentary goes into the US entry into the war in greater detail

I can really only shed some light on the second part of your question based on some recent reading I did. In The Great War of 189-, written in the early 1890s, the authors speculate a Franco-Russian Alliance fighting against the Triple Alliance of Germany, Austria, and Italy. Additionally, England joins the war on the side of Germany, notably different from real history.

In The Angel of the Revolution from 1893, a Franco-Italian-Russian alliance is pitted against a German-Austrian-British alliance. I'm sure you can see some patterns here, some of which are true to reality, others (like the UK siding with Germany) are not.