We would highly suggest trenching and treating instead of only treating inside the cinder block wall. Termidor is meant to bond to soil and be applied into the ground. It will not last very long just treating into the cinder block walls. Cinder blocks are also very porous, so they will soak up a lot of the termidor and you might get a few months of protection instead of 10 years when applying it into the ground.
You should follow the instructions as outlined in our How To Do A Termite Treatment article to treat the exterior and underside of your crawl space.
We cannot answer that question for you. You are definitely not required, however, if your state requires a termite letter at the time of closing, only professional companies can issue these letters and if you do not have an active contract with a company you will usually be forced to pay a large sum to have your house treated to get a clearance letter. You should call your state Department of Agriculture which is usually the department that manages pest control companies and ask if your state is one that requires a termite closing letter when selling a house.