1) How does parking at a hotel (for in-city ones with restricted parking) work if it isn't your name on the room? Can you secure parking at the hotel, or do you need to park elsewhere in Indy?
You simply park. Give them a room number, a guest last name, or tell them you need to check in. It's that simple. If it's a downtown hotel expect parking to be full, though.
2) How does one pay the person who has their name on the room? Partially up front, all up front or all on the day in question? Cash, check, or app?
As someone who has attended GenCon for 14 years, and has been rooming with a stranger for the past 4 years (my name is on the reservation), I require full payment up front from the tenant or the tenant doesn't get a room key. I ONLY accept payment via PayPal for goods and services.
Make sure this person is paying you with a secure app like PayPal and make sure you know their full first and last name, with phone number. Also get a picture of the reservation for proof. Or switch and reverse if you're paying them.
3) How is the receipt of room keys coordinated? Are they picked up at the hotel desk, or exchanged face to face?
I meet the person who is staying with me face to face, shake his hand, talk to him, etc. and then hand over the room key. I usually go up with him, talk about beds, determine shared space, etc. and that's pretty much it. This is where I set extra ground rules if I didn't clarify before. Like no coming in drunk at 4 AM and vomiting.
4) How much luggage/swag is acceptable to keep in a shared room? Last year my total load was 1 suitcase, 1 backpack, one mid-sized cooler, and a 24 pack of coke, would this need pared down or partially stored in the car?
Everything you can fit in your designated area is all yours. If you're paying, I would expect the person to be cool with that as long as your stuff isn't all over the room or in his space.
5) Are there any matters of protocol/courtesy re: sharing living space with strangers in general or at GenCon in particular that would be beneficial to know in advance?
Most people don't seem to mind. As long as you're not in their way and aren't trying to follow them around you should be good to go.
Bit of a bad story: Keep convo light. I stayed with a guy who would not stop talking at night and kept me up for like 2 extra hours. He also waited until I left the following morning and started going with me to the con. This is not a buddy hang out or like, pay for a friend. You're there on your own, sharing a room. Needless to say I never stayed with that guy again.