nvu

remove the lower dash, should be only Philips screws, unhook some switches attached to the plastic panel. then if needed you can also unhook the fuse box from the frame, it's only some 10mm bolts/nuts. no need to unhook anything, but should give plenty of room

you have the version where the 2 bolts are external to the tank. i don't think it needs any washers. middle one. the one i have the bolts are internal to the tank, directly touching the fluid. the washers go inside between the bolt and the internal tank bottom. right one. rtv wouldn't hurt, just give it a day to set. it really shouldn't need any to seal. thinking that bottom plate is tweaked, check that it's flat and square. If the crush washers helped, maybe the bolts are too long and not original. Here's the link to the pic https://parts.subaru.com/a/Subaru_1996_Impreza-Outback-Wagon-22L-MT-4WD/_54100_6028409/POWER-STEERING-SYSTEM/G10-346-02.html

If it's just at the reservoir I'd rtv the mating surfaces. The copper washers are for the newer style reservoir.

push the piston 80% of the way into the block, tighten the compressor a bit more, tap around the edges so the compressor gets into the chamfer, tap the piston in

are you using the ratcheting sleeve compressor? place the piston face up on a flat surface, place the compressor over it and lightly tension it down making sure everything remains square. pick both up and put it them on the block, tap it so the skirt protrudes into the block and the rings are around the last tension band. lightly tap around the top of the compressor and tension a bit more. make sure it's squared and centered. the piston should slot in with moderate tapping.

Center diff takes the brunt of the mismatch. In a safe place, drive in figure 8's and see if there are any odd noises under the car. Center diff failures are usually a binding/release feeling, then quick knocking sounds, then catastrophic failures that could lock all 4 wheels. It's probably safe if there are no noises or binding.

For composite gaskets it wouldn't do much as they fail the rubber/graphite breaking down and eventually coming apart. Moving the thermostat to the top has the risk of it never opening if there's a pocket of air big enough to dry it out.

No

R180 only found in sti, and mated to the 6spd. Will need bigger axles and the bigger hub knuckles from an sti of that era. To drop in and replace an r160 you'd have to frankenstein some axles and possibly slot out the driveshaft flange. I'm only familiar with the gc/gd era up till 2007. Maybe later models have something what would drop in.

it's a bit misleading, they are and aren't depending on the years. 05+ sti's changed to 115x5 hubs and cartridge style bearings. They are no longer press in bearings. 08+ wrxs changed to cartridge like sti's. so, 04 sti, 07 and earlier wrx are exact same knuckle/hubs as base impreza. no differences there, only beefier axles or different brakes on the better models.

Cut up a credit card, jam in between seal and snout, pry all you want with screwdriver.

Might be a weak tensioner, it shouldn't move at all when the engine is running. Half a tooth off on one side isn't a big problem.

From the 02-03 factory manual

Another vote for tensioner

Not sure if it's necessary, but move the cam and crank sensors over too. One less thing to troubleshoot.