Does that mean they can actually disable the IME?

What I do is use scrcpy to navigate apps that aren’t navigable via remote. It works pretty well. Just turn on wireless debugging in developer options and adb connect. Then, type scrcpy, and you can control the thing with your laptops mouse and keyboard!

You can also just run Onenote in a virtual machine.

Yes, for someone with limited Linux experience, Unraid is a better choice than Proxmox. And, the experience gained through configuring Unraid will be applicable should you want to move to Proxmox later.