verifying voltage and reading the computer. Sounds like the guys you have looking are not fully troubleshooting the issue. My guess is it’s an electrical issue of some sort.

Check the relay and connections at the relay right next to the pump or did they only look at the ones under the hood. Did they clean all the contacts? A good mechanic would have traced power all the way to the pump and through all other components. If the pump works, did he check the bank one 02 sensor to see if it was good?

Could be lots of things but they have to follow through on all the basics at this point to figure it out.