1. 2 inches is an INCREDIBLE amount of stretch. Something is wrong.
2. What are the targets stuffed with? Arrows bouncing off?? Again, something is wrong. You don't say if you are just starting out, but a 55# bow, IF putting that energy into the arrow, should "stick" the arrows.
3. When you unstring a bow, let the string slide down the upper limb, don't take it off.
4. Yes leaving it strung will help "set" the string. But, again, you are getting too much stretch.
If you are just starting out, 55# is a LOT of weight and I wonder if you are getting to full draw. Just a thought.
More information is needed. Pictures of the bow string at the limb tips would help. Pictures or better video of you shooting would help.
Arne