U codes

Communication codes can be difficult to track down depending on what tools a shop has. Usually you need to scan all of the modules to determine which ones are online and if one or more isn't, then go to the one(s) not online and test to see if they are offline because of an internal problem or if it is the communication buss that is the issue. In this case there are multiple modules that have lost communication so it could be a power issue as well that is causing it. Over all I would say diagnostics would take 1-2 hours or so if the shop has to dig but it depends on how good they are with diagnostic work or if they farm out that work to someone else.

It depends on what type of code it is. These all should cause some symptoms if they are current codes. I would probably write them down, then clear them and see which ones return. They could be caused by a loose or faulty power feed or a module that is intermittent and that is what could take a while. Most of them will set if the communication is lost for more than a few seconds, so even a loose connection could do it.

I use a scan tool to clear them as that way I'm sure they are gone, plus if they come right back I can quickly scan again to see if the issue causing them is now there. I don't think you can clear them without a scan tool as most on board systems let you see the codes but not erase them, that is to keep people from clearing a code prior to sale or inspection. Unless they changed it the on board system won't tell you if they are current or history codes, but at least one has to be current if the CEL is on as history codes generally turn off the light after a few drive cycles where they don't return.

Yes, you want the engine off while filling as the system could decide to run an EVAP test and draw a lot of fuel vapor or raw fuel into the system and cause problems. Not just triggering the light but it could draw fuel into the EVAP canister or the lines. Replacing those because of the extra fuel in them isn't cheap.