No, it is not possible to remove a disk without rebuilding parity, and of course, rebuilding parity would make it impossible to rebuild any data disk. At this point, I think we can forget about rebuilding anything since it seems extremely unlikely parity is even valid.
If he ran with a missing disk for some time he hasn't even been able to do a parity check, since parity can't be checked in that state. All it would do if you tried is a "read check", which doesn't do anything except see if the disks can be read.
New Config is going to be the way forward. Probably disk2 is OK. Maybe we can get some data from the original disk1. It doesn't sound like any of the other disks are expected to have any data. I can only assume disk3 was added with New Config, since otherwise it wouldn't let you add a disk to a new slot when you already had a missing disk. Possibly disk3 is unmountable because it was never formatted and so has no filesystem.
@beno, you didn't actually answer this question: