I have a 6MT 2006 TL with 210k miles. Original owner and it's always had meticulous maintenance. Never given me any major trouble until now.

Recently I've noticed what sounds like an occasional single cylinder misfire with accompanying vibration while at high idle after a cold start. Noticeable between 1100-1400rpm and smooths out outside of that range (to include at low idle). No codes. I don't have a scan tool that can monitor individual cylinder misfires but I checked fuel trim and bank 2 is noticeably richer than bank 1. Short term trim on bank 2 is between -6 and -10% while bank 1 is +/- 2.

I did compression and leakdown tests. They were both performed on a cold engine and dry with the following results:
Cyl #1 - 187psi / 6% blow-by
Cyl #2 - 187psi / 6% blow-by
Cyl #3 - 200psi / 11% blow-by
Cyl #4 - 175psi / 4% blow-by
Cyl #5 - 180psi / 30%% blow-by
Cyl #6 - 190psi / 12% blow-by

I wouldn't infer much from the compression results except that none of the cylinders have catastrophic compression loss. I was only looking for big picture results so I wasn't cranking to max compression and some of the cylinders might have gotten a few more or less compression strokes during the test. On cylinder #5 leakdown I don't hear anything escaping into the intake or exhaust, but there is considerable loss through the crankcase (confirmed at the dipstick).

For context, the car never burned a drop of oil until about 120k miles when it started burning about 1.5qts per oil change interval (5000miles). I just completed its 2nd major service: timing belt, water pump, spark plugs, valve adjustment, and I resealed the oil pump as it was just starting to weep. All the spark plugs came out looking perfectly normal. I know everybody will assume I botched the timing job, but I've triple checked and everything is properly timed. The engine did not rotate while the belt was off. I'm confident the valves never touched pistons and the leakdown test points to piston rings. Additionally, I noticed the rough high idle on cold starts maybe 15k miles ago but never looked into it as I usually get in and go, and by the time I come to the first stop, the car is warm enough that idle is down below 1000rpm and it's smooth. I looked into it now as on the heels of the timing job I was tuned in to anything that sounded off, and also had to let the car idle for an extended period (bleeding coolant).

To me the good compression on cylinder #5 with bad leakdown says that the cylinder walls aren't shot, but rings are likely stuck. Does that sound reasonable? Any good way to distinguish definitively between the two? I have a USB camera that I stuck into the cylinder and everything looks fine (no excessive carbon), but it's not a purpose-built borescope and I can't get enough light to see cylinder wall condition.

Interestingly, if I move the #5 piston to a few degrees before TDC and run the leakdown blow-by is reduced to 20%. Still no good, but considerably better. Initially led me to assume it was valve related, but given that I don't hear anything in the intakes/exhaust, a worn spot in the cylinder wall or a function of piston rock-over?

The car functions just fine at the moment, but I imagine thinks will spiral if I just ignore it (foul plugs and converters, stuck rings scoring cylinder walls, etc.).

So I guess my questions are:
Is this problem common on the J32?
Has anybody had success freeing stuck rings by soaking pistons? What solvent did you use?
If soaking pistons isn't viable, is it worth pulling the pistons and replacing rings? I'd do both banks if I went that route. Can the bank 1 cylinder head be removed with the engine in the car (no lift or hoist)?
Anybody have a recommendation for a rebuild kit if that's what it comes to?

The rest of the car is in great shape. It'd be a shame for this to be its end and I'd honestly trust this engine rebuilt much more than a used engine with an unknown history.