Thanks greatly to both of you for the suggestions/insight. This car probably has several issues, but it's probably best I focus on one at a time. For now I'm going to focus on the misfire.
Now that I've had a chance to calm my frustrations, collect my thoughts, and reassemble everything again, here's some further details. Some of this may or may not be relevant, but I'd rather give a plethora of useless/redundant/unrelated info than not enough. So story time.
I feel fairly confident that connections weren't missed or properly rejoined. To change the tensioner I removed only the radiator/fans (for working space), belts and A/C belt tensioner, harmonic balancer, and timing cover. Regardless, I can't discount having dislodged something by mistake, so I took Texan and rasterman's advice and spent a good deal of time going over hose and electrical connections. The exception here being that I should probably check/reseat the spark plug wires. Poor connection(s) may have come loose in the course of my work.
I replaced the plugs and wires about a month ago (what a PITA as far as plugs/wires go), and I never felt fully confident that the wires went on properly. Everything seemed to work, however, so I left them alone. It's also my understanding that if a loose plug wire was the cause of misfire, the ECU will throw a misfire code--which it has yet to do. A misfire without a thrown code, from what I've been reading, points to a mechanical issue. (please do correct me if this is inaccurate)
In my haste yesterday, I never let the engine get up to operating temp before losing my cool and tearing it back down. I immediately assumed a slipped tooth upon hearing it run for the first time. Now that I've gotten to letting it properly warm up, driving it around the block a few times, and carefully listening to it, I notice a few things.
It definitely seems to be a misfire on one cylinder on every revolution. Very rythmic and consistent with RPMs. It can be heard popping/puffing/(backfiring?) from the tailpipe. This was not at all present prior to the timing belt change. I have yet to pinpoint which cylinder might be the culprit. I'll update this post after I look into it.
Once the engine warms up fully, it has a very difficult time idling. It seems to want to stall, but never quite does. It shakes quite a bit and the miss is much louder (maybe cause of lower RPM?). Giving it some gas, the miss is still readily apparent. The car, overall, is able to get down the road just fine. It's noisy from the misfire, but aside from that seems to run smooth.
There is also a very audible tick/knock (some audio might be in order here, I'm terrible at describing these things). Using a stethoscope, it seems to be loudest on the valve cover near the intake for cylinder 4 (nearest to driver's seat on LHD.) It could be lifter noise for all I know, but it seems too loud and 'clunky.'
I removed the intake chamber to get a look inside the throttle body and intake manifold. There's some carbon buildup and oily residue. The oily residue also seems to have been erupting out through the throttle body opening into the intake chamber--there's a messy splotch opposite the opening in the chamber. The small hose attachment on the bottom side of the intake chamber, near where the hose from the air filter and MAF sensor connect, is pretty messy with oil as well.
As suggested (thanks again), my next order is going to be checking out the PCV and giving everything a good seafoam treatment.
As someone who's inexperienced with Subarus and cars in general, I'm more than happy to read and learn through experience. I encourage everyone to correct me as much as possible.
Other notes:
Prior to tensioner replacement, OBD was showing my timing advance being (IIRC) 16 deg at idle. Post belt change, it's sitting at around ~28 deg at idle. I won't pretend to understand how the ECU controls this--some reading is probably in order on this. May or may not be relevant.
The CEL has not come on since tensioner replacement.
I pulled the crank position sensor, and it did indeed have a very thin layer of what looked and felt like metallic dust. It cleaned right up, and I reinstalled it. This didn't seem to make any difference.
I do have an OBDII scanner, so if any information from that might be useful, I'd be happy retrieve and share it.
