I've heard the British wanted the Bourbons - not sure why
I've heard the Austrians wanted the Napoleon II (makes sense because his mama is Austrian)
I've heard the Russians wanted either due d'Orleans or a Napoleonic Marshal (elsewhere I heard they were OK with a republic) - not sure why
AFAIK, Alexander did not have any clear idea on that account and I'm less than sure that he seriously considered any of the Napoleonic marshals. Bernadotte was out of the list well before the Congress of Vienna and never was considered quite seriously by Alexander (why bother if he was quite convenient to Russia as ruler of Sweden) or other allies (for whom he was still a byproduct of the Revolution) even if for a while this prize had been held before his nose as an encouragement

)). But as you know he figured things out, left the allies and concentrated upon getting Norway.
As for the republic, IIRC, he just expressed an opinion that everything including a republic would be OK as long as the French are happy.
If it is true that Russia found the idea of a French republic preferable to the Bourbons, or even just tolerable, why was that outcome anathema to any of the other powers, like Britain?
My impression was that the Brits just wanted to roll things back to the pre-revolutionary times. Besides, Talleyrand managed to assure the allies that the Bourbons are the only plausible solution for France. Taking into an account that Britain was pretty much a paymaster of the coalition and that Alexander was something of a hysterical anglophile, the British voice carried a lot of weight.