Stop heating the mouse in the microwave. You are probably partially cooking it, and snakes aren't designed to eat cooked food. This also probably changes the smell, and that may be why your snake doesn't recognize it as food.

It sounds like you do not have an adequate hot spot, and over all your temps are too cold. This can lead to several problems, including feeding refusals, regurges if it does it, and respiratory infections. If the coldest it gets is 78, you are probably ok as far as regurges and RIs, but the lack of a proper hot spot could be related to the feeding refusals.

If your snake is still little, a 20g tank may be big an intimidating for him. You can try cluttering it up with extra hides, crumpled newspapers, or fake plants to help your snake feel more secure. Also, make sure that 3 sides of the tank are covered.

Finally, when feeding, it is a good idea to prescent first. This means let the snake get the smell of the food for about 1/2 hr before you offer it to the snake. This way the snake knows food is coming and will be ready and waiting. This is especially important when trying to convert a live eater to f/t.

If you do all of this, then wait a week for your snake to get used to the changes, you may find that your snake eats just fine. If not, I'd suggest feeding live until you get him eating consistently, then trying to make the switch to f/t.