Welcome to agility! I'll try to answer your questions.
I'm not, but I'd suggest volunteering at a few events and soon you will know lots of people. Most agility clubs are very friendly and always looking for some helpers.
I used to, but have cut back and have stopped doing USDAA. Due to time and $$ constraints, I cut back and dropped USDAA from my list. (At one time or another I've done 3 types of agility, 2 types of coursing, conformation and therapy visits with 2 different dogs pretty much at the same time). I don't do USDAA (although my first agility dog got his AD) since my 17 inch basenjis would have to jump 22 inches. They don't have any problem, when I train at 22 inches, but it is just easier for me to just do AKC and NADAC where they only jump 16. And yes, I know they could do performance, but it was just easier to opt out of USDAA. I would suggest you try them both out and see what you think, but I'd start with NADAC if you can. There are some differences, but many people enjoy both.
I belong to a club, but they don't offer classes (other than the ones I and someone else taught last summer). I've also found I'm a bit beyond most of the classes/trainers in my area. I would suggest you stick with the classes for as long as you are getting good instruction and don't have all the equipment at home. I had great trainers before we moved.
I think it depends on the dog. I've found my basenjis do best with me. Other dogs would do fine with anyone running them. I've run a friends' dogs on ocassion, but no one wants to run my badsenjis . My husband was supposed to run the tri basenji, but Jet just totally blew him off. I'd like to see them try again as my husband is really a decent handler, but we'll see. It is really annoying to have your dog run out of the ring after 2 obstacles.
Any time. Will be interesting to see what other people's experiences are.