Good

Healer says:

E Commit: this attack is returned to its printed damage. if your opponent's character has a greater HT than yours, you do not to block this attack to be able to play a Reversal to it.

1. if my opp's Chain throw have a 15 of damage, and i active Healer to return to its printed damage, can my opp increase that attack again?

2. if i active healer and i don't have a reversal attack on my hand, can i active the effect without reversal on my hand?

1. Yes, it'll go back to its printed damage (normally 5). there's no restriction on the opponent raising the damage again (like Fight or Flight) so if they still have damage boosts available they can use them.

2. The reversal effect is not manadatory, you can play the ability without having a reversal in hand. There's nothing on the card that says you must have a reversal to use it.