2000 Cadillac Deville Will not start when temp gets below

You are dealing with the coolant level sensor that has nothing to do with starting. It is just coincidental that it starts sometimes, and not others. I would say, when it is cold and wont start, spray a little starting fluid in the throttle, see if it tries to start. Unless you have a fuel pressure gauge to hook up to it. You are either losing spark or fuel, it is a little easier to determine fuel first.

You have a pressure test port on the fuel rail, no need to remove filter for check if you have the right fitting. If you have adequate pressure(40-45 psi), then it could be a temp concern. To validate this at first, a scan tool that is capable of reading data would be required. Although, if you disconnect the coolant temp sensor, open circuit reading to the ECM is -40. When it won't start, disconnect the coolant temp sensor, and see if it starts. The thought behind this, maybe it is not reading cold enough when it gets to the temps your experiencing. The coolant temp sensor is in the rear bank head(1, 3, 5, 7) at the trans end. If you look down below the master cylinder at the rear of the head, you will see a conduit that leads to some grounds on a stud. Just to the left of that ground stud is the ECT sensor(two wire YE/BK). Another concern might be some moisture in the fuel that is icing causing blockage or restriction. We don't typically have temperatures that low in our region, so I am just giving some ideas.