I can't recommend restricting the return air flow. Have you found the main system filter? That should take care of your needs. 4 hours to reduce the temp 5 degrees is not, in itself, an indication of a problem. Was that at a very hot time? Neither is cycling on and off for a 50% cycle. Running continuously on a hot day is not an indication of a problem either.

Please consider posting some pictures of the nameplate on your indoor and outdoor units.

I am afraid that if you can't get the landlord to fix the system, your best option might me to get some window units. That way, at least at night, you could cut your electric consumption by cooling only the bedroom with a relatively efficient unit. Be sure that your electricity is up to the task before you buy.

Prior to attempting these filters, the system kept running and running. It did this so badly that it ran out of freon and the coil froze.

Has nothing to do with the return size and location. Your supply ductwork is insufficient, unable to get enough CFM to get the cold off the coil before it freezes.Also makes sense why it has to run so much to cool the house, there isn't enough supply airflow.

Btw, your AC system does not "run out of Freon" from being overused, the Freon is recirculated and goes through an evaporating and condensing phase which transfers heat. Systems lose Freon through leaks that can develop over time.

It sounds like your home had floor furnaces at one point. The large return air grilles sound like old openings for floor furnaces, used a long time ago. Where to start?

First, your system should have filters somewhere in the return duct or inside the air handler or furnace. I would look really good to make sure you haven't missed something. I've seen filters placed in really weird places. Sometimes filters aren't found for years. By then they're pretty stopped up. As far as the return grilles being large, the larger the better! One thing that kills your system and your energy efficiency is duct work that's just too small. Modern systems are made to run with very little static pressure in the ducts, around 1/2 inch. That's not much, so I wouldn't worry about the large ducts. The filters are another story. You really need a filter in your return duct somewhere. If there's not one, i would install filtration. Usually the cheapy filters work best as they cause the smallest pressure drop in the duct.

You mentioned the refrigerant leaking out. If you have a refrigerant leak it should be repaired. That stuff ain't cheap. Besides it's illegal for a technician to keep adding without repairing the leak. If someone is having to add refrigerant, you definitely have a leak, no question about it. If the service provider you called did not look at your duct work, did not tell you your system had a leak, and did not offer sound recommendations, please ditch them and find someone qualified.

As far as your system coming on and off every ten minutes or so, that's called "short cycling" and isn't good for your system, your energy costs, or your comfort. It sounds almost like your system is either too large for your home or you have massive air infiltration. Either way, this needs to be addressed. Really, I would recommend you find a good service provider in your area that can be trusted to offer technically correct advice. I usually find it best to ask friends, co-workers, or other family who they use and trust. In the meantime, you may want to check out a couple of articles I've written dealing with heat loads and your comfort. Here's one HERE if you would like to read it.

Let me know if there are specific things I can help with. I'll be happy to help if possible.