A few months back I came home from work and it was scorching hot in my apartment. The fan would not cut on, but I could hear liquid (presumably refrigerant) moving through the pipes in the wall. I turned the until off just so I wouldn't freeze anything up. The next day, my maintenance man tells me that he added more freon and that should clear it up. I knew that wasn't going to fix the problem, but as the unit seemed to work again after restarting, I let it go.

After this incident, I changed the thermostat myself (I've done this in every apartment I've lived in for the last 4 years). The old one was a traditional mercury one, I switched out for a digital one. Everything was peachy after the new thermostat was installed. Before I changed it, I could hear the refrigerant moving through the pipes whether the fan was on or not (really, it would run all day with or without the unit blowing air).

Today, the first 100 degree day here, I came home to my apartment scorching hot again. The fan is running non-stop, and the air coming out of the vents is cool. Since I am hugely pregnant right now, I demanded my maintenance guys come over right away. After replacing the air filter, this is what insued:

1. I was told it needed freon. (if it needs it again after only 3 months, find the leak!)
2. He couldn't figure out the digital thermostat and thus blamed it. (guy #2 knew it was a good thermostat as said it was not the problem)
3. their conclusion was that the return was not in a good place and that they should cut a hole in the wall to make another return. More intake equals more output, according to them.

But, here's the kicker- they didn't even open up the inside unit to check the condenser or blower fan! They are supposed to be coming back tomorrow to install another return and a new vent cover that will push the air that IS coming out down so I can feel it. How logical is this solution? How should I go about keeping them from cutting a hole in the wall without actually checking the mechanical condition of the A/C unit?

Sorry to say but you are dealing with the idiot mentality of many apartment/condo service people. If the unit worked at one time then why would the unit need more return air now? I have NO solution to the problem because you are dealing with people who could NEVER get a real job at a HVAC company and those are the people who want to try and fix your unit. Call a real expert OR be stuck with what you have since that is the only two solutions you have available to you at this time.

Logic says if they freely admitted you are low on refrigerant then they should find and fix the leak first. But since you are dealing with the Keystone Cops of HVAC, you will probably have to fall back on hvac1000's advice.

I am curious if you are even low on refridgerant. Sounds to me like they don't know what they are doing in the first place. Maybe you are overcharged now.
I agree with finding another company, at least for a second opinion.