What was the actual code you got for the crank sensor (P0XXX code)? Just because the code says crank sensor in the description does not mean you definitely have a bad crank sensor. Codes give you direction, not answers.
Given you ran out of coolant, I would do a block test to make sure the head gasket is OK. You may have had enough coolant to cover the coolant temp sensor, but not enough to pump through the system.
For question 4, do not ever put a stop leak product into the cooling system. Sure it will plug leaks, but it may also plug small passages in the cooling system.