Will one or several shut down during filevault decryption harm things?
You can safely use, shutdown, sleep, and restart your Mac during the FileVault encryption and decryption process. The process happens in the background while your Mac is awake and plugged in.

FileVault encryption or decryption can take hours or days to complete, depending on your disk size and type. You simply have to be patient and let it finish. Portable Macs are required to be plugged in to decrypt or encrypt, so keep it plugged in and on if you want it to complete in a timely manner.

Your encryption or decryption will pause when asleep, shut down, or unplugged, and it will resume when awake and plugged in.

See the following for more information:


Apple Support: Use FileVault to encrypt the startup disk on your Mac
University of California: Use FileVault to encrypt your Mac
Backblaze: All about FileVault: Encryption for your Mac
University of Victoria: Encrypt a disk using FileVault 2: Mac


The question about your mouse and keyboard issue is, at the time of writing, very unclear and may be better suited for another separate question. It's highly unlikely that FileVault will have an impact on your built-in keyboard or trackpad.