Rob, one thing that kills off a lot of home citrus here in Florida is known as "foot rot" and it usually occurs when a home citrus tree is subject to overwatering, often by sprinklers watering lawns. So maybe just gently clear soil away from the trunk and check for any cracks or weepy areas in the bark near the base. I hope that's not what caused the die-back event because if it was, it's probably going to do it again.
Going forward, (I was waiting to see your picture) my opinion is that for the size of the plant now, you don't need a bigger pot but Daisy's advice regarding drainage is good. Maybe just prop up the pot with stones or blocks of wood so that the bottom of the pot doesn't sit directly in the saucer? Then you can see how much water drains through the pot when you water in winter without having to clean up puddles.
I also agree that you need to get yourself some citrus specific fertilizer if that baby is to grow and prosper. We fertilize citrus in the ground here 3 times per year, February, May and late September. Your action to remove any fruit trying to start is also a very good idea - maybe let it grow out for at least another year before let it make another fruit. Those little plants have to put a lot of energy into making big lemons. Enjoy the flowers, then pinch off the button.
Other than that, keep doing what you're doing and keep that baby warm! Our winter temps here in Florida are pretty much as you described - 70's daytime and 50's at night. Any less than that is going to cause the plant distress. Even though these Meyers are described as "hardy to 20F" that doesn't mean the plant's going to grow and bloom and get big and lush if you regularly subject it to 20F. It might survive, but . . . that's not what you want here. That's the fatal flaw with the whole "hardiness zones" system. A plant tag that says "zone 4 to 9" means the plant will survive zone 4 and zone 9 but it really will only be happy in zone 6 or 7.
Btw, if you could give us a more specific location than your zone, that would help us to advise you going forward. Seems like maybe Alaska?? Even Alaska has microclimates though. Near the coast is a lot different than inland, right? So if you'd fill in your profile with the nearest city that will give us more information on your year-round climate.
