A) In a lot of places this would take US pressure and forces being directly involved, since in Sub-Saharan Africa there was no significant desire for independence among the elites, with the occasional exception like Madagascar. Among the common people there was discontent, but not enough to make it so that those regions will simply easily separate from France. It is going to be messy, take lots of effort, determination, and painful concentration from the Americans. Algeria of course, will be the most painful of all. Any French government would be committing political suicide in 1945 to force colonies which do not want to be independent to become independent, when they are being given an ultimatum to do so. Most colonial territories in Africa if they do go independent, will be closely aligned with France, just like OTL (possibly even more so with Algeria). Although, it the political ramifications that I speak of in C go through, then that might not be the case.... much the comfort that that will be for the United States.
B) The United Kingdom is going to be fanatically opposed, as will all other European colonial powers (Netherlands, Belgium, Portugal, Spain) who will be horrified and outraged because they'll see what the US is doing and realize that they're next. An important reason for why the UK supported French membership on the Security Council was because the French would side with them on colonial matters and they were worried about being diplomatically isolated. This is infinitely worse. The UK might be the one with the biggest vested stake, but the other European colonial powers viewed their empires as extremely important : The Netherlands pinned much of their post-war recovery hopes on it, and the Portuguese were obsessed with the matter. Expect a very different and much less close US accord with these countries post-war.
C) Domestically in France this is going to be nightmarish, since it will be perceived as being an attack on France, and everybody will oppose it : there is no political party which will support the idea of French colonies being forced to be given independence by the US. Every moderate and conservative party will be heavily discredited, the French political elite will be humiliated, extreme opposition to the United States will result, and the already fragile French political scene is going to implode.
The absolute best that the US is looking at in such a scenario is a separate and independent European bloc which is opposed to the US on security, colonial, and defense policies, and is probably essentially finlandized by the USSR. The worst that they're looking at, because the European powers don't have the capability to do that in the immediate post-war era, is the victory of French communists in their elections, which will almost inevitably cause an Italian communist victory since the Italians were not that far off from that as well, and thus the effective collapse of any hope for a united anti-communist political bloc in Europe. These countries won't be Soviet puppets, they are too big, developed, and not conquered by the Red Army, but there won't be any equivalent of NATO, and it is hardly impossible to imagine the remaining territories on the European continent in Germany and the Low Countries being untenable and collapsing. After all, the French and Soviet occupation zones in Austria and Germany are going to leave precious little of UK-US regions intact, and UK-US cooperation might be somewhat... awkward. A Soviet-aligned bloc stretching from Brest to Vladivostok in effect, and almost certainly with any remaining capitalist states on the European continent finlandized by it.
Which is the inherent problem with most of the proposals to use US might to grant the colonized nations independent in 1945 : the European states are much more important than their colonies at that stage, and the US annoying them carries intensely negative consequences in the context of a bipolar world. Its possible that the US might have been able to get away with breaking off Indochina as is sometimes suggested, but the rest of the French colonial empire is going to take catastrophic costs for the US to break up in such a way.
As for the decolonized nations, you're probably looking at French West Africa and French Equatorial Africa gaining independence as federations rather than single states in 1945. I don't know about the French Caribbean, I presume they would gain independence as singular islands, but perhaps they will be grouped into a single Caribbean bloc. Most will be even more politically unstable with even less time preparing them for independence (which isn't an excuse for the Europeans and their conduct of decolonization, just there was some time OTL which they spent propping up local leaders before independence to ensure stability post-independence, which will not exist here), and will be riven with political disputes over their independence and course. The only savings grace they have is that it took more than a decade OTL for mass-level political movements to start to become dangerous in most of the states, and so mass level political action will be less developed and less of a threat to political leadership. But there will be much less direction and unity, and its entirely possible that some of these states might melt down, to profit of communist factions.
