Well I just wanna finish up the story of this issue, in case it will help anyone else coming across the same questions!
Long story short, it ended up being a problem with both the timing belt AND the camshaft sensor. I don't know exactly how those two things may have been related to each other in the moment, but here's the longer version of what happened:
I checked the timing belt shortly after it was suggested I do so, and laughably, that definitely seemed to be the problem as when I opened up the covers, the timing belt sorta...flopped out. It had broken in half. Which frustrated and confused me, since I had changed it less than a year ago! This time, I ordered an actual Subaru brand timing belt (I *had* done my research to buy the last one, and decided on getting the whole kit from a brand - I can't remember it now but I'll look it up - that was recommended to me as being just as good as getting Subaru parts!), and a friend and I put that belt in and also discovered that the tensioner roller seemed to have seized. That bushing that the bolt goes through was just....stuck, somehow. Crappy part? We're not exactly sure, but luckily I had a new one waiting, and we put that in along with the belt. The piston tensioner and everything else was fine.
So we put everything back together and presto! The car started, sounded fine and normal, and I was ecstatic! My friend went on her way and I drove to the store later that evening -- where, halfway there, the same thing happened as back when all this started. Very suddenly, the car died, no rough running, no weird noises, just noped out and the engine shut itself off. Luckily I was on an only mildly busy street and managed to pull to the side as it was happening. Tried to start it again and it was an absolute no-go. Starter motor tried, but no crank. So this time, I didn't sit there with my heat-addled brain trying to get it to start over and over, and just got a tow back home. Once there, I notice the check engine light is actually on (a rarity in this particular car), and I get the codes for coolant temp sensor, camshaft position sensor AGAIN, and knock sensor.
So my friend comes back over the next morning with all her toolkit and we look at the sensors. The wiring looks fine, and suddenly, she's able to start the car and it runs at idle without dying. We poke and prod the wiring harness, can't kill it. Then we wonder about the signal getting to the ECU, whether it's just weak, or intermittent, or what have you. The harness FROM the ECU to the sensor tests good (thankfully - my car for whatever reason does not have that big harness connector at the firewall - the harness goes through and into the dash and there's not a damn thing you can do about it LOL). We test the sensor itself but we don't have an oscilloscope so the best we can do is see that it IS getting some voltage when we swing it past a large piece of metal. Then we look into the wiring order itself, since neither the sensor nor the cobbled-together connectors I put in have the original wiring color scheme (according to various manuals.) We decide to swap colors (in this case, the sensor has red, white, and black. We know the black is the shielded one so that goes to yellow on the harness, which, incidentally, goes back to a large black wire where it pins into the ECU). Just for funsies. Because the other two colors on the harness are white and black, and the other two colors on the sensor are white and red, so what if white goes to white, like...ya know...makes sense? So we do that and immediately get some new information - car does not like that AT ALL. It starts, it runs, but it makes angry noises and won't go above 3000 rpm. So, just to be scientific, we take the sensor all the way out to confirm that yes indeed, the car will not start without the camshaft sensor (I mean, ya gotta try, right? LOL)
So luckily, on a whim, I had ordered another sensor off of EBay (because parts stores want to charge me $300 for it and at this moment I'm not exactly made of money, which is why I'm doing all this myself with the help of cool friends). We manage to find a wiring diagram that includes the SENSOR wiring (I wish I could tell you where, I think my friend still had access to some super-secret mechanics' manuals from when she worked at an auto shop) and discover that it goes like this, for anyone interested who has this particular setup: ECU big black wire (shield) --> harness yellow wire --> sensor black wire; ECU white wire --> harness white wire --> sensor RED wire; ECU small black wire --> harness black wire --> sensor WHITE wire.
So with the new (used) sensor wired in like that, we start the car up again, everything runs peachy keen, I drive around town a bit with my friend following me in case anything else happens, and the car runs like a dream, except the check engine light comes on again!
I drive home, check the CEL code, and it tells me it's the knock sensor...........which I had forgotten to tighten down way back at the beginning and it was completely loose 😂
That was that, for now, in this saga! As of this moment my car is running like a champ with no check engine lights and we're hoping it stays that way! Hope any of this info ends up being helpful and/or interesting to someone, thanks to everyone here for their help and suggestions!
