not realy since you sitll shoot back just as bad.
If boht you and the enemy have disadvantage, the relative situation remains identical
No, it depends on the number of attack rolls that are disadvantaged for each side. If you have 2 ranged attacks per turn, and your opponents have 7 ranged attacks, you are better off dropping prone since statistically you will have a better damage ratio per turn if you do so. It will, however, lengthen the number of turns taken to complete the combat.
EDIT: This is a simplification that assumes you are superior to the enemy but outnumbered, since the chance to hit and damage done per attack by you and your opponents needs to be factored in to determine if dropping prone is a good idea. Disadvantage makes the likelihood of hitting non-linear, which changes the way you need to look at damage per turn calculations.
