I'd back up a bit. The original issue was a "cranks but won't start" complaint with a communication "U" code pointing at the FPCM correct? Knowing what that "U" code is would be a big help. A dead data line can cause a lot of issues depending on the modules involved, possibly even keeping a module from going to sleep which could explain the battery draw issue.
Without a lot of equipment to work with, a quick and dirty check would be to connect a 12v incandessent style test light between the positive battery post and the positive cable (cable removed), key off, doors closed. If the light is on and "bright" it's a good indication that you have a high parasitic current draw issue that's pulling your battery voltage down. Leave it attached and watch it for several minutes to see if it ever dims down as a module that may be awake goes to sleep. All that's going to tell you though is that you do or don't have a high draw. I think that if you know, or could find out what "U" code was set that led you to replacing the FPCM, and what modules besides the PCM had codes it should help you find out what's going on. Modules need power, ground, and communication. If your FPCM has power and ground I'd start running checks on the data circuit involved and associated modules