If Sweden joins the war before the Russian surrender, could they ask for Finland in Brest Litovsky? And if they do and the russians give them, could Sweden crush the finnish resistence, or they would have to compromise?
I concur with Lalli above. There is no incentive for Sweden to try to take Finland or "crush the Finnish resistance". It would be a lot more trouble than it would be worth. Finland, after all, had almost half the Swedish population, and by 1918, there were many militantly nationalist Finns to fight any occupation, let alone by a nation that had not engaged in a war for a century and thus would be squemish about organizing and running the harsh occupation needed to subdue the Finns.
No, Sweden will not aim for retaking Finland, not under a sane leadership. I find it unlikely that even a
Fascist Sweden would see it worthwhile to outright conquer Finland. But after WWI Sweden might push for a Swedish king and the cession of Åland like my compatriot says above, in exchange for Swedish support for Finland shaking off Russian rule. Down the line, keeping the Swedish political, cultural and economic rights in Finland at a high level would be at the top of the list, as well as paving the way for Swedish companies and capital at the Finnish market. But otherwise, an independent and
friendly Finland is the best bet for Sweden, as a buffer against what ever Russia could become after the Great War.