One place I lived in had a no pet policy, which included every type of animal. I didn't live there long..
I just want to mention something in case you do go ahead and upgrade. Don't put a betta AND a molly in a 2.5 tank.. that is no where near enough for both.. mollies can get 4-6 inches, and you really shouldn't put 2 fish in anything under 5 gals (even then it's pushing it).. So.. if you're able to upgrade (which I hope you are for the sake of the fishies hehe), then really consider not putting in another fish that can get 6 inches in something so small =( And that includes a divided 10 gal since it will be the same amount. Major over crowding and bio load.
Other then that.. good luck! But since you make it sound that she put her foot down.. then be careful, because I really don't think you want to go homeless. Much as we all love our pets, the way the economy is now, you really don't want to lose your home.
Mention they are small fish too. Some people think Oscars when they hear the word fish. Ask as if you don't have them... then you can gauge by her answer if you need to take action or if it will be fine.
It sounds like they talked to her since he was calling her names.. I mean, she may of turned an eye if he had one, but 7 individual tanks (or even 3) is hard to ignore for some people. Especially if down the road she wants to sell the house, any less risk of messing it up will be her top priority.
With regards to the unexpected visits being illegal, check the lease agreement and local laws. Sometimes it is completely within the bounds of the law.
I feel the landlord issue though. I have the same "no pets at all allowed" condition in my building because some of the tenents that lived in the building before me had cats, and they completely thrashed the units they were in. I've asked around and know a few people have some fish, but haven't heard from him personally that it's cool or what the conditions are. As a result, my tanks are 1g each so if he does raise a fuss, I can just say it'd be no different if a vase broke or I knocked over a pitcher or something. Still might have to get rid of them, but I think he'd be reasonable on the matter--provided I'm not asking for a whole-wall-sized tank.
I feel for the landlord too though. If you fill a tank too full, near some electronics, the water spills over...could cause some issues...
Regardless of what happens, I sympathize and hope all works out!
Edit: as someone else alluded to, specify "small fish" or "those little Siamese fighting fish". A few bettas is a lot different than a large pacu, arawana, or saltwater tank. Or, if all else fails, see if there'd be any conditions under which she'd change her mind on the policy, and do what you gotta do. Anyhow, hope some of this helps!
Topics like this make me so thankful I have my landlord. He knows I have the dog and cat, but didn't know about the dragon, rats and tanks. When he came over to fix the hot water heater, he had to come into the living room. He saw every pet we had, I was sweating bullets. Then he stopped and looked at the rats, commented on how big their cage was, looked at the Dragon and asked if she bites. He said he would love to have one, but his wife won't let him have pets. Then he saw the betta tanks and went nuts saying how great they were set up, their tanks were so big and how much time and care must go into them. He then saluted me and walked off. That was 2 months ago, he never said a word since. He pays the water bill hehe
Some provinces here cannot say anything about a caged animal or fish in a tank in their rentals, it's law that you cannot remove someone because they have a caged pet. I have never come across a landlord that has an issue with caged/tank animals. I hope something can be done about your situation. 3 tanks is much easier than all those small ones!