Therefore, it wouldn't make much sense to send the change to the sending address.
true, but when they designing software that will send change to 'old' addresses it would make much much much more sense that sending address is used instead of second key in the wallet, i mean, come on
IMHO both versions make not much sense, I suspect it was a limitation of Multibit Classic to keep the number of generated keys low. MultibitHD uses a new address for change as it should.
If you checked all backups you might want to search the disks of the computer(s) you use. Data stored is commonly not really deleted until its overwritten. Maybe some of the lost keys can still be found. Its something I would let an expert do though.