I haven't used multiple vets yet (still figuring out when my pets will go so after that I'll get a different vet to do the tick/tapeworm treatment).
I would do what you listed...check with Nebraska USDA people that it's OK that multiple vets do the procedures vs. paperwork, then check with the Baltimore vet to make sure they have documentation that all of this happened
I guess the two tricky bits would be:
- will your dog pass the bloodwork given that the vaccine was given a year ago (I would think yes, but I'm not a vet

)
- you'll need the vet who signs off on the paperwork (presumably in Nebraska) to verify that the microchip/vaccine were done in the right order and the bloodwork test results are fine...that means a Nebraska vet would have to sign off on the Baltimore vet's work...you may want to check with the relevant parties and make sure they're all fine with it
Also, check with the Baltimore vet and see if they can send you exact records proving the chip/rabies...my USDA vet said they needed documentation of the date(s) involved with all of this (though then they never checked the paperwork I brought

)
Assuming all that checks out, though, and it should if everyone's USDA approved and you have documentation for all the steps, you should be fine.
Sorry that was really long winded...Next question quick...After you go through the blood testing and 6 month wait ONCE yadayadayada...does this mean that your dog can travel multiple times to the UK if you get a new certificate each time you travel and you never let his vaccination period lapse? Is there a maximum time (ie 5 years) as to how long you are allowed to wait to get a certificate to travel?
As Smocha said, once you're in the UK you can get an EU passport which makes things much easier. FWIW, though, my USDA vet claimed that you could renew the paperwork/skip re-doing the 6 month wait for 2 or 3 years. I don't know if that means there's an actual "time limit" on the bloodwork or not, but that's what he said.