Runs on a/c all the time no heat - 1997 Ford Explorer AWD

Have the cooling system pressure tested and see where it is leaking.

check for an over heating situation with the motor as what you say indicates engine over heating.

Are you adding coolant when you let the air pockets out ? Sounds like its still low on coolant.

How do you know both heat and a/c run at the same time? Usually a/c is on a separate switch. If you say you can turn the a/c on yet run the temperature up to hot and get heat, yes, my VW jetta works that way too. If you think you have a problem, I would go ask a dealer. Usually determining if you have a problem or not (are the controls working as expected) is free. Repair, if necessary, costs $$.
Given the age of the car, it is not impossible there could be a problem.

you probably don't have a thermostat in your car

Change your temp sensor

Sounds like you have a stuck thermostat. New or not, thermostats can get stuck.

Yes that is cool if it's correct. 195 F is approx.as low as you should have. I'd change it. Be sure to run with cap off, and heater on (not defrost) while refilling system. Wait for rad fan to come on 2 or 3 times before you put cap back on, then add coolant to return bottle as needed. Make sure rad fan only comes on when up to normal running temperature. If it runs all the time, it may be causing low coolant temperature too.

check the fan on the radiator it should start before it over heats