First partial answer.
Compound Bows . adds the propierty of Massive Critical which adds a 1d6 dmg on criticals aside the damage of the arrow . this has to be crafted and it destroys the original Bow, this ones can't be applied to Drop items just the crafteables ones
On the compounds parts ( that is how they called but they are the ones who apply mighty ) .- This ones are "enhacements" to bows and Slings, that allows you add the propierty of "Mighty" to your bow. On this there are 3 levels you can get which gives your bow , or sling respectively the Propierty Mighty +1 , +2 or +3, this means that the arrows you shoot from that bow will be able to add up to that number stenght bonus to each shot, this means that if you have a char with STR 16 and have a mighty +3 bowl, each arrow that hits will add 3 more damage to each shoot, if the very same character has str 18, the damage will be the same. The diference is that this are applied upon the bow and no crafting process is needed for the adding of the porperty ( just having the bow you want the mighty applied and the compound part of the adecuate level ). This parts
can be added to all bows included drops, and yet there are some drop bows that have it already alongside some nice buffs.
About the levels think that
This link may be able to help you.
Cheers
JC / Redeemer