Heating pad on low setting with a towel between baby and the heat should be fine, depending on the temp in your house. She shouldn't feel cold to the touch, nor hot when you touch her. If she is being kept too warm she will get dehydrated. You can check if she is dehydrated by "tenting" the skin on her shoulders. Just lightly pinch it up and if it stays, she is dehydrated, if it falls back into place she's doing good.
You should be able to tell if she is full by looking at its milk belly. If you see air bubbles in the stomach (im guessing shes a bit transparent still?) Then you should try laying her on her stomach and "burping" her by massaging her softly. But, generally she should be fed every 4 hours. Even during the night.
The last 2 questions seem like normal baby behavior, but the baby may have some kind of trauma that we don't know about, you won't be able to tell until she gets a bit older.
Maybe put a small stuffed animal next to her so she feels like she has some company and something to nestle up against.
Sadly, Chances are, she will die eventually, if she is actually 6 days old.
Another alternative is going into local pet stores and see if you can order a lactating mother mouse. And then just hope she will take care of it, if you really want to keep it alive.
Good luck! Post pics if you can.