Would 7 gallons of water be my target total volume to get me down to 5 gallons of wort?
Depends on your pot size and flame (surface area) thats a good start (something you can tweak as you go)
Salt is included in the example BIAB recipe which is a schwarzbier, I don't think this is needed in my recipe, right?
I wouldnt over complicate your first AG...personally I would skip it unless your sure about your water chemistry.
What is the minimum acceptable temperature I can drop to during the 90 minute mash?
I would be happy from 154 for a start and maybe heat it up if its start getting around 148 (rember keeping it simple for first brew...but if you are anal say maybe 150
How would you calculate efficiency in BIAB?
With Brewing Software
Figure how much Potential gravity points you should have..... and what you actually ended up with (math...or promash) But you have to be sure of evaporation (finished Volume math) and fine tune it. Its all about tweaking your system.
But YMMV...just rember to not over complicate your first AG. As long as your fermentation and sanitation are up to par, I'm sure the beer will be great either way.
Also be very carefull that bag does not touch the bottom of that pot....or you will be looking for a strainer....(go easy on heat with that bag in there...even up the sides of your pot)