As I've said before, Ashram's a scary scary dude. Making your final battle with him a Big Crossover Event could accentuate it's importance in your character's story.
Thus we are back to Andy as a 17 year old. I'm thinking for the moment, then, we not write him into “Opening Night” and have, as Andy's own prolog details, Venkman bribe him later to come to California to train the GBWC. Andy decides he likes it there and sticks around some, dividing his time between ETGB and GBWC, until the final battle with Ashram. The “Andy in a wheelchair” subplots can be used after that (I know, quite a ways down the line).
Otter, I'm not sure what I'm trying to say either. Most of our characters meet for the first time in “Opening Night” (episodes 1-3) and the follow-up (I tentatively title it “Learning Curve”, episodes 4-6, where Andy will be brought in, Robert, Chad, and John Lipsyte recruited, maybe leading to the bust of Ravisher in episode 6. By then we know he's only the tip of the iceburg) Dr. Masterson is a character with a rich backstory built in, but one thing that springs to mind is this: why would some guys working in LA recruit a guy living homeless in New York? Brainstorm some of that. Does Joey just trip over you while on a visit to Ghostbusters Central? Or does say Ray Stantz find you in an alley, and knows who you are (“Wow!!! Doctor Otter, inventor of the RAM drive!!! Awesome!!!”); when the GBWC sets up, he goes “I know who you guys can get to work on your stuff…” (knowing full well there are some other tech guys like Robert, but it's a big operation). Maybe none of the above. But sometimes having a bunch of ideas to discard helps find one you like… :d .
I'll try to get some revisions to “Opening Night, Part One” uploaded shortly (a more complete description of Ravisher at the end.)