1 Lucky Texan

Dorman sells a box of 20mm washers but, yeah, you could just buy a several next time you're at the dealer too. https://www.amazon.com/gp/product/B001SNRGAA/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

if a few sprays of starter fluid calm it down - that would point to fueling.

On my WRX I run Kumho ECSTA ASX or ECSTA 4x .

if the bearings for the toothed idler are failing, that could be a source for magnetic debris too.

fix the headgasket, sell the car, buy a turbo Forester to play with.

I have read of some negelected engines having gummed-up oil control rings. Might cause low compression?

I dunno, wondering if examining the plugs might give a clue? maybe a cylinder is misfiring and a plug would appear different than the other 3 ?

sweet ride! Make sure to get OEM or Japanese parts for the timing belt system service.

many of us would use Valvoline MaxLife trans fluid in that car. I did. It's Dex II/III/VI w'ever compatible.

rear bushing on the lower control arm can knock/thump.

downstream (post cat conv) does nothing related to a:f or other parameters related to the engine's performancein that car (pretty sure only the H6es may use that sensor for a:f corrections). It is a 'nanny' for cat conv heating-up properly. hopefully, someone with more experience will replay but one thing you might do is, in a new thread, ask for recommendation of a soob-friendly shop near your city. Someone may know a good independent mechanic.

many folks have good luck with the Subaru Coolant Conditioner (repackagaed Holt's RadWeld) ONLY one bottle, ONLY install per directions.

hmmmm....OK, next time it's cold/prone to exhibit failure....try cycling the key several times, from OFF to ON-wait for a 3-4-5count, OFF-ON,wait,OFF-ON,wait, then OFF- Start. If there's leak-down of pressure in the fuel rails, that should correct it and maybe starting will be better? wonder what the plugs are like? proper NGK or some oddball type? just trying to eliminate some 'problem areas'. any stored codes?

If it's coming from higher up, oil pressure sending switch/sensor might be leaking. They can leak a lot. kinda under the alternator?

yeah, I get it, my situation was more, 'open'?, and I had splines. Though, in line with your comment, maybe that meant MORE rust/corrosion was involved? I never even got anything red so, just watch out for fire. You may need to either spray a LOT, or wait until parts cool a little. Can't se much opportunity for liquid to penetrate if it all vaporizes! anyway, fingers crossed...

make sure the car is idling on level ground when you check the trans fluid level.

hmmmm, with cool weather, a bad engine temp sensor might explain the symptom of initial lean/poor running with better bahavior after warm-up. Can't signal the ECU for 'choke' injector settings if it thinks it's already a 'warm' engine.

dead coil? I think the cyl pair 1&2 share a coil due to waste spark.

I think I would try 'moderate' heat on the bolt, then spray/flood with some penetrating fluid. I had read before that suggestion , the fluid cools the parts and as they cool, the fluid can be 'draw in' . Then let is sit and go back to trying to drive the bolt out. I might try it twice before going back to a more drastic plan. I did a similar thing to a severely stuck axle-in-hub problem on a chevy impala. Propane heat for 30 seconds at 12 and 6 oclock positions, sprayed with PB blaster, waited a coupla hours I think, went back and it started moving finally. ymmv

maybe a pinhole is spraying coolant on a cracked spark plug wire?

hasn't ATF has been 'synthetic' for a long time ?

Wait, is this a SUBARU dealership? If so, should be easy to get documents showing what parts have been put on the car - if they serviced the timing belt system, you want to see more than just the belt listed as mentioned above. The toothed idler is often the point-of-failure, not the belt. maybe push them for some little powertrain warranty?

ATF = automatic transmission fluid

^^^ what he said sounds good, maybe check for a particle of metallic debris on the sensor too.