[Laminar]

If you install a Megasquirt, you generally remove the factory ECU completely - it can no longer get mad about O2 sensors if it's not wired up.

The Megasquirt does not require that you use any wideband sensor, it can accept anywhere from 0 sensors to one per cylinder depending on how much info you want. But I can't imagine the nightmare of trying to tune a car without a wideband - you have no idea where it's running and you're either leaving tons of power/efficiency on the table, or you're running way closer to dangerous conditions than you know.

 

[Laminar]

The PimpXS is a Megasquirt board inside of a stock-style ECU case that plugs into your stock wiring harness. The normal Megasquirt kits would come in their own case and require you to custom wire the kit to your car. I'm running an Innovate MTX-L on one bank and a Spartan on the other bank.
I drilled my exhaust and welded bungs in place. The bungs are easily plugged if you don't want to keep a sensor there. You can also get clamp-on O2 bung assemblies if you don't want to weld to your exhaust.

 

[Laminar]

Correct.