Ok guys, i'm willing to bet that this has been covered before, but i just gotta ask again. Ok here goes: i have a '99 4-door 2WD. i love it. Well, i replaced the fuel pump about 2 1/2 months ago. Drove great. my cat started rattling and couldnt afford a new one do i cut it off, and just ran it without for about a month. It was really loud, but it ran just fine, then the wife complained enough that i just straight-piped it to quiet it down. Ran it one day(about 3 hours of driving) with the stalraight pipe on it. Next day i drove it about 30 minutes, then all of a sudden it just started running like crap! Sputtering and stuttering, stalling out, loss of start-off power and massive fuel consumption, with a really strong gas smell. SES light came on and started flashing. so i had autozone run the codea and they said O2 sensor bank 1 sensor 1, and P0300 random/multiple misfire. its kinda old, so i changed the plugs, wires, cap, rotor, and new O2 sensor. Still misfiring. So i then changed the distributor itself, no change. Mechanic buddy has a 2000 S-10 same motor, he says ECM went bad. So i got a used one, had it programmed to my VIN then did the security relearn, on it( but not crankshaft relearn, as ive read on here) and its still misfiring! replaced a few vacuum hoses that were old, still misfiring.

Things ive checked/replaced:
EGR valve: not stuck and went ahead and cleaned it while i was in there.
Unplugged MAF to see if change in performance: no change
Plugs, wires, cap and rotor
Distributor
Vacuum hoses( the "T" on drivers side just in front of firewall)
O2 sensor Bank 1, sensor 1
Fuel pump & filter

So my question(s) is this: 1) does anyone think the misfire is due to the straight pipe? Can i just remove it and it will run better than it was? 2) not having the crankshaft retard relearn procedure done(but nit throwing codes for it) FUBAR myself and my blazer? 3) only other thing i can think of is a weak coil, but i always thought they just went out, not got weaker. Any and all ideas, tips, suggestions, tricks, are greatly appreciated. but please keep in mind that im just a mechanic when i absolutely need to be and dont have a scan tool or a pressure tester or anything special-just two hands and a few wrenches. I love my blazer but am at the end of my resources!