Thank you for rescuing this little gal. BTW what is her name?
I'm puzzled as to why your vet is waiting to spay and dental. You said b/c of the weather, how hot is it? We have temps in the 3 digits and they do spays with no problems. As for a new dog, my vet does rescue dogs all the time for foster homes and just before they go to a new home. As long as blood work has been done before hand I wouldn't worry too much about the surgery. However, there are no guarantees with any surgical procedure. Sounds like your vet might be concerned with the discharge possibly being Pyometra, here is a link
http://www.sniksnak.com/doghealth/pyometra.html and that is something I would worry about.
I've had dogs and cats as old as 14yrs have dentals with no problems. I would have both procedures done at the same time, as you said you don't want to put her under twice. If she is having problems with her teeth perhaps she should have a round of antibiotics. I know I had a very old dog and we did antibiotics every so often b/c he could not be put under for dental work.
If you are not comfortable with this vet and his diagnosis, you might seek a second opinion.
Good luck and please keep us updated.