Lost Cause S40?

[SlowBMW]

Long-standing BMW guy, I bought a Volvo S40 to be my daily in these harsh Chicago winters. When I bought the S40 it had some issues like an occasional cyl 1 misfire and it needed a o2 sensor (bank 1, sensor 1.) It seemed like the previous owner just drove it without maintaining it, but the oil was full and had been changed within the last 1,500 miles so I offered him $1,000 under his asking price and explained why and he sold it to me.
Since purchasing the car, I have replaced 5 ignition packs, 5 spark plugs, the MAF sensor, the crankshaft position sensor, and changed the oil with some synthetic oil rated for the winter temps.

Today, I took off the intake manifold and cleaned the throttle body and MAF since I am still getting a P0101 code, put in the new CSP, and extracted an intake mani bolt that the previous owner or mechanic snapped, and tried replacing the o2 sensor. Turns out, I got the wrong o2. So, a little disappointed but still happy that I got to fix and clean some more small items on my S40, I put it back together and my buddy hopped in the car with me and we took it on a 20-minute drive with my scanner hooked up and all was well. No codes, other than the p0131 for the o2 I had hoped to replace today. The S40 felt responsive and good. Got back to the garage and let the car idle for an additional 10 minutes as that's where the car previously had the most trouble (holding a steady idle and not bouncing) and it was still a little wonky but significantly better than it was a couple of hours prior. Fast forward about an hour, and I am about to make the several block voyage back to my house thinking the car was in a better standing mechanical condition than it was, and I start it and I have the worst idle I've ever seen. Pretty much nonexistent without my foot on the throttle. I drive the several blocks back and it was incredibly clunky and I had my CEL flashing at me and the "Slow down or shift up" message on my dash, with the code p0300 for multiple misfires. I'm about at a loss with this car, I don't know what to even diagnose at this point. All of the parts I've replaced have been OE/OEM (Denso, Bosch, or Genuine Volvo) and I'm still having trouble.

TL;DR
I had P0101 so I replaced the MAF sensor with a new Denso one, code still persists
I have P0131 and tried to replace the o2, but was sent the wrong replacement
The S40 was running better this morning after I replaced and cleaned parts, then an hour later the car hardly moved under its own power, and P0300 popped up. All new ignition packs and spark plugs within the last 100 miles
I don't know what to do

The car is a 2004 S40 with the NA 5cyl B52 engine
Any insight is appreciated