I don't know.... I don't have any experience of 30 minute nap, resettle and then another 30 minute nap. For us, 30 minutes has always meant OT but I'd be surprised if he then did another OT nap after being resettled? Is it definitely 30 minutes, not 35 or anything?
I know you want to get out of this mess ASAP but you could just write off the next few days, up his A time by 10 minutes and see what changes and go from that? If it doesn't fix anything at least you'll know where you are. If there's not no change it probably means he's OT cos if he's UT an A time increase would probably help, even if only temporarily? I know it's really hard doing that but I've always found the torture of not knowing whether J is OT or UT more frustrating than the short naps themselves!
What happened when you picked an A time and stuck with it for a few days? Did things improve and what happened next?