MultiBit got progressively slower after adding more receive addresses
It seems very unlikely that a number of addresses as small as 120 could make a significant difference in performance. I would guess there is some other problem that just happens to coincide with you adding addresses.

The inability to open the wallet, and the excessive disk I/O, is particularly troubling. One possible explanation is that the hard drive is failing - the slow I/O being caused by repeatedly retrying failed operations. I would start by backing up all your files and especially your wallet (which you do regularly anyway - right?). If that fails because there are files that the backup software cannot read (without a good reason, e.g. they are locked by another process), or it reports I/O errors, then that would clearly indicate a failing drive that needs to be replaced.

(Generally, if you have good, regular backups of your wallet files, you are fairly well protected against "catastrophic money loss".)