Have a 1999 Toyota Avalon with 52,000 original miles. All of a sudden, started it and it was running rough. Definitely missing.

I was on a 110-mile trip to my parent's for Thanksgiving. Car ran perfectly until I stopped to pick up groceries two miles from my parents house. When I started it at the store, it ran rough and was definitely missing. Never had a problem of any kind with the engine before. Has ONLY 52,000 ORIGINIAL MILES!!! I use only synthetic oil and car has been babied. My uncle lives down the road from my Dad, and he has one of those diagnostic computers. He tested it and said the problem is in cylinder number one. The three ignition coils each cover two cylinders, so he said it can't be that. The plug wires and plugs are original, so I replaced the wires. Did not help. Then replaced the spark plug in cylinder 1 (back side of engine on passenger side, correct?), but this did not help, either. I am 110 miles from home in somewhat remote area. There is an auto repair shop five minutes from my Dad's, but don't know if he knows much about Toyotas. Won't be open until tomorrow. Talked extensively with my mechanic of 15 years. His opinion of the potential problem: head gasket, valve problem, cracked head, piston problem, or cracked block. Given low miles on engine, great maintenance, and durability of this engine model, unlikely that it's a cracked head or block, but anything can happen. Said it could be some weird problem we haven't thought of. Said take it to Toyota dealer 25 miles away for opinion. Could be something simple. But, should I drive it in this condition? If bad head gasket, coolant could get into oil and blow engine. Could just stop every five miles and check oil for signs of water/coolant. I don't have AAA and don't have much money, so I'd rather not spend $210 to rent a U-Haul truck and car dolly to tow car 120 miles back home to my mechanic. Any suggestions or advice would be greatly appreciated! Thanks!!