1) The ecu runs the engine. It runs the efi system, the spark control system, and the idle system. Code 11 may or may not be the cause of your idle problem (likely not) but no ecu power means no engine operation is possible. If the car won't run or strands you somewhere idle is unimportant.
2) Your guess is completely wrong based on (my turn to assume) the continuing death spiral of science education in America.
3) You can be sure by measuring it. That said, if it's not the two wire sensor it's not the one for the ecu.
Fix the code 11 first. Clear it but keep the diag jumper in there. Jiggle wires around until it returns. Check the cluster of grounds on the intake manifold, the EFI fuse for corrosion, and all the other paths that supply power to the ecu.