Try and just answer in order without writing a novel:

1a: exFAT/FAT32 work just fine everywhere. NTFS works fine from Linux, but do to it sucking, may eventually lead to corruption. Ext4 works from Windows with a plugin.

1b: There are very few Windows programs that you can’t find a Linux alt for, and Wine does work on almost everything. Few exceptions would be from the developer of said software intentionally making it difficult. Adobe suite (soon to be fixed) is tricky, some kernel level anti-cheat games won’t work online, and some corpo software with crypt locks may be tough. There are emany simple Wine managers like Bottles to help make this dead simple.

1c: Firefox profiles are fully portable to any other Firefox install.

1d: No. Every media format is covered. This is not an OS thing though, this is an application thing. I can’t think of many apps that use proprietary local data formats anymore. You’d be better served asking about something specific.

1e: Nothing. It doesn’t touch any of your filesystems unless YOU touch them. Don’t delete anything, and you’re fine. It should even automount your existing identified partitions for you to browse through.

1f: “Viruses” and other malware don’t really exist on Linux or MacOS because of the permissions structure. Your regular user doesn’t have permission to alter the global system without a password. Don’t execute random code by giving it that password, and you’re fine. Your regular operates in its own sandbox, which is your user profile. Anything stupid you do as that user is just localized damage to that user.

1g: Very few things won’t work, and it’s likely to be some small production run variety of something. A cheap components by an unknown manufacturer with Windows-specific interactions is about it. Just stick to well known manufacturers, and do your research first. Even then, in time, most things get support if there is a large enough consumer base for that device.

2-3: I wouldn’t even bother trying to figure any of this out, because Microsoft constantly changes their mind about this, and they’ll soon just force you into this abomination of Windows 12 they’ve been talking about recently.