Welcome! If you can keep your warranty then go for it. Otherwise, I'd go and look around at a few options out there just because of the recalls and other issues the EcoDiesel's are having.
It happens, not with just EcoDiesels.. the general consensus is to not do it because you have someone is taking over your truck and doing god knows what with it.. I personally would never go for it.. but that's just me.
Welcome! That's the dreaded code which seems to have multiple fixes. I'd start by the methods you mentioned, then searching around a bit on this forum as we've got TONS of info around this issue.
3-5 MPG from a plug in module? I've been around diesels since the first 01 Duramax and never seen one of these work that well.. Did you hand calculate the mileage?