Say you have a seller who's firm on his price but comps suggest $50k lower. Do you extend the lease term to 2 years or 3 years on a CA just to get the deal done and give the seller closer to his target figure? If so, do you still advertise rent credits? At what % or rent?

Steve, everything is on the table with a CA. Length of lease, rent credits, purchase price, etc. Whatever you and the homeowner can agree to. Of course, the key being when all is said and done, will you be able to find a t/b with those terms you have set.
Each deal is different, of course. But with regards to rent credits, my usual formula is 50% for 12 months, 33% for 18 months, 25% for 24 months, etc.
I've also set up deals at 50% for 12 months, with the second 12 months receiving no rent credits. Again, it depends upon what you and the homeowner work out.