I'm concerned that you're talking yourself into this buffymad, when I have to say, you've identified so many reasons why this would be a bad idea.
I'm rather surprised, and a little concerned that the animal shelter that your husband works at would allow him to take on a dog, knowing that no-one will be home during the working day.
I work for the RSPCA, and I hope that the shelter isn't an RSPCA one - as all potential owners (including staff) would be subject to a home visit. Leaving a dog unattended for this period of time would surely result in a failed visit and no dog. The vast majority of animal rehoming charities now follow this practice also.
I know it's not the advice you would want, and that your heart is in the right place. But unless you can make arrangements for the dog in the day, please reconsider taking him on.