FileVault Taking Days To Encrypt

Six days is a bit long even for a traditional hard disk drive, and suggests that it may be operating in a state of failure. I recommend investigating that possibility.

De-crypting will take just as long as encrypting.

My MacBook is unusable ...

A Mac's performance will be somewhat degraded while either one is in progress, but not very much, and certainly not to the point it becomes unusable as you wrote. That supports a suspicion that the drive has failed. They can't be repaired; they can only be replaced. You can have Apple perform the replacement, or you can do it yourself. It's not particularly expensive or difficult.

Presumably you already created a Time Machine backup: How to use Time Machine to back up or restore your Mac - Apple Support. The ability to restore a Mac's content from its TM backup makes the inevitable disk failure a non-event.