Welcome Chewie!
Here's a nice online ASL dictionary with phrases:
http://www.aslpro.com/cgi-bin/aslpro/phrases.cgi
ABC dictionary:
ASL Browser
As for the decrease in an ability to communicate by writing, what do you mean by that? For deaf people who have dementia/alzheimers? If so, I'm not aware of any studies in this area.
I don't know about Deaf Chat Coffee in Oklahoma. You probably could send the host an email asking about this. My opinion is that all hearies are welcome -- but would you spend a long time chatting with a foreigner who barely knows your language? To get best results, I suggest finishing your ASL courses and if someone thinks you are fluent enough -- get a good dose of self confidence and a sense of humor -- and you're set to go to a deaf event. I don't know anyone who really have fun 'working' at a deaf gathering (ie. teaching people who are learning signs, etc). I could be wrong, though. I would feel weird if people came to the deaf events just to stare at me signing like I was in a zoo.