Venture van started running rough and barely starts

First have the computer scanned . Most part houses will do it for free. The miss fire codes will tell you where to look. It can identify which cylinders are miss firing or not firing at all. Identify the bad cylinders and pull the spark plugs and run a compression test on them to make sure they are ok. No compression means a problem like a burnt valve. Make sure that the firing order of the plugs are correct.

it sounds like timing skipped a tooth

I Think Your Problem Is Still With The Fuel Supply,Have You Changed all Fuel Filters,Follow All The Fuel Lines Make Sure There Are No Kinked Fuel Hoses,Some Times You Find Small Filters In The Top Of The Fuel Injectors.
Hope This Helps

did this occour suddenly? if you can get someone to turn the A/C on briefly while your looking under the bonnet and look at the A/C pump. if you dont know what one it is wait untill they turn it on and find which pulley stops turning. because it sounds to me like your A/C pump has seized and if it engages and the pulley stops turning that would confirm it. once you know you can still drive it just dont turn the A/C on untill you get the compresser replaced

The vacuum line on top of the EGR is required for propper vehicle operation. The vacuum line is what opens the valve when the solenoid is comanded on. As for the MAP sensor check the injector/MAP sensor harness right by the driver side corner of the rear head. The harness tends to melt and cause rough running conditions and misfires. So would the EGR which is a common issue and yes the the whole assembly should be replaced.

Could be your belt or the tensioner.

Testing is the way to a fix . Fuel pump pressure testing , electrical testing . Viewing engine sensor data on a scan tool . Instead of guessing as to the problem learn to test or take it to a qualified repair shop . Changing ECU without doing it correctly can cause problems that were not there before . PCM - engine computers need to be programmed , dealer only.
GM Fuel Pump Relay Testing
Testing Electric Fuel Pump Problems with Voltage Drop Test
048 Engine Hard to Start or No Start Scan Data for Starting Problems

remove dog house locate test port and install guage should be 45-60 psi

take a look at the fuel pump

this vehicle has coil packs for the ignition correct? have you ran the engine at night with all surrounding lights outs so its really dark. spray a mist of water on the coil packs and spark plug wires to see if you have spark/voltage leaks jumping out from them. sometimes the tiny sparks will be so fine and skinny you can barely see them so it needs to be dark. hope this helps your trouble shooting. a coil pack could have such a fine crack for the voltage to leak from that you cant see it.