How Exchange Online automatically cares for your public folder mailboxes (via AutoSplit)

brlgen I don't have a clear answer for your 1st question since it's out of Microsoft support recommendations however answering your 2nd question is on the same article for migration with PFs greater than 2GB we might need to modify the value for DefaultPublicFolderIssueWarningQuota  & DefaultPublicFolderProhibitPostQuota over Exchange online organization configuration to have greater relative values over the migrated PF size, 

answering your final question Autosplit will be triggered but it shall skip that PF as it's a Giant PF.

It's better to split that PF to smaller subfolders to have a better migration throughput and better management on EXO, for more information over migration best practices please check this article.