Only if you've got it cranking all day. I've got a couple of Tiny (they're Micro, which is the same thing) systems that are silent when idle and nearly silent when running less than a load avg of 5. It's only if I try to spin up a heavy, CPU-bound process that their singular fan spins fast enough to be noticable.
So don't use one as a Mining rig, but if you want something that runs x64 workloads at 9-20 watts continuously, they're pretty good.
First off, if your clients can play the files natively, then anything capable of running Jellyfin will work. You only need specific hardware if you're going to transcode streams.
If you need transcoding, especially if you're going Intel, anything 8th Gen or newer will work.
As for your Linux distro, use whatever you're most comfortable with. Ububtu LTS or an EL-clone are both popular choices.
If you're replacing all of O365 (excellent choice, BTW), I do recommend Nextcloud with a few plugins. I use it specifically for sharing contacts and calendars among my family.
LibreOffice is my desktop word processor and spreadsheet, and I use it more than OnlyOffice, but if you need two people in the same file at the same time, OnlyOffice is a better option.
If you're (going to continue) using Office 365, you can use Evolution as an Outlook replacement. Evolution EWS rides OWA and ActiveSync protocols to give you email, calendars, contacts, notes, etc. I've used it for over a decade. It works very well once setup.
As for Android, there are several, including Outlook for Android (which is bloated and slow, being a Microsoft product), which I am forced to use because of our company SSO config.
If you're looking for an Office 365 replacement, I use Nextcloud for my personal stuff. It has files, contacts, calendars, notes, etc. If you install the OnlyOffice plugin, you get multi-user online document and spreadsheet editing. I use the DAVx5 connector to get (shared and personal) contacts, calendars, and tasks in my Android phone. It integrates into the environment so all calendars and contacts apps work automatically. It also automatically backs up pics/vids I take with my phone automatically.
Not really. Windows only supports FAT and NTFS filesystems natively. There was an old ext-fs driver back in the day, but I have not looked for one in a decade or more. There might be one out there already.
The deal with case-insensative support is likely from Windows users who are annoyed that Readme.md, readme.md, and README.MD are separate files on ext4 but the same file in FAT or NTFS. UNIX and Linux come from a school of thought that allowed you to do things like use different case in filenames.