I know that I can't activate (for example) 5 Morty Cards and get an extra 5% interest but can I activate 5 Morty Cards and get 5 months of the extra 1% interest?

I don't think you can activate more than one at a time, not sure honestly. But they're broken right now so if you activate one you should have the extra 1% forever, since they aren't expiring like they should.

There was recently a site update a week ago saying they fixed the buff problems and now the cards and the in vault buffs would be working and expiring properly.

Huh, I missed that. Well in that case I would probably just wait but I honestly don't know what would happen if you tried to activate more than one.

I just redeemed another with one active. Although it did take the card, my buff page says that it still Expires in 6 days 22 minutes 56 seconds, so I'm guessing they don't stack that way either. [edit] Aaand I didn't wait long enough to post. Now, it's saying Expires in 4 weeks 1 day 23 hours 51 minutes 47 seconds.

edit 2 Well, Hell, I don't know. It just dropped back to the original time. Sorry guys, I tried to help :(