You probably have a leak, it happens. I assume wtih the hunidity the AC has not been keeping the temp down as well?
1. It could be frozen up, but you would notice the ice on your coils. Could be something in the coils or condensor. But your best bet is a small leak somewhere in terms of cost
2. Sure, its not hard
3. The check should be cheap. <100, but he is probably gonna be like well your AC is empty, so I gotta add 6 pounds at $50 a pound. So $100 for the call, $150 to fix the leak (if small) and $300 to fill it up. Or something like that. There are 2 types of refrigerant. R-22 (expensive and old) and r-410 your outside unit should have a sticker saying which. Sometimes the leak is in the coil itself so it has to be replaced ($500 or so on top)
4. Probably not, just a leak.
Age of your unit is probably getting up there. Id put it at >$500 is the replacement line. A new AC unit and coil should be about 3k where I am. Get a few quotes or better yet find someone with rental properties and ask them who they use. Im not an HVAC guy, but I own lots of them and they break all the time. Needless to say I got a good guy who works on my AC's and I am probably funding his kids college.