If your hCG levels go done for a threatened miscarrige can the hCG levels go back up? Can it be a healthy pregnancy?

Possible but unlikel. Usually laboratory errors can do that, or if you did the pregnancy test at different facilities. However if done at the same place and properly its a bad sign and miscarriage/ectopic pregnancy are likely, sorry!
Hcg dropping . Dropping HCG in early pregnancy usually is a poor sign meaning that the pregnancy is not developing properly. While you can choose to take another test to be certain, I have not seen cases where it again starts to rise.