Sending the contract does not lock you into the the rate. You have to give them permission to lock. You will also need to sign disclosures saying you want to move forward with them as your lender. Even if you do all of this you are under no obligation to get the loan thru them as all you would need to do is send an email saying you have decided to use a different lender.

Reality is, all lenders are going to have about the same rate and fees. If someone was that much better then everyone would used them.

Your best bet is to find a broker and go with them. Brokers typically have a little bit better pricing than mortgages companies (such as Rocket), credit unions and larger banks.