1. This thread covers what you are asking:
https://www.dakkadakka.com/dakkaforum/posts/list/732247.page
Short answer, yes.
2. In the case of multi wound models, after you have determined how many wounds are inflicted, your opponent assigns one wound at a time (starting with any injured models), determines damage for that wound, then moves onto the next.
So with your above example, you scored 4 wounds, with d3 damage against a squad of 2 wound targets, I will lay out a potential scenario. Your opponent assigns the first wound, you roll a 1 for damage, that model receives a wound. Your opponent must assign the next wound to the same model, regardless of what you roll that model will be slain and excess wounds will be wasted (only mortal wounds carry over). He picks another model for wound 3, you roll a 6 scoring 3 wounds, the model is removed and the extra wound is lost. The 4th wound is assigned to a 3rd model and you make you final damage roll, which will result in either a half or fully dead primaris marine.
3. Only if the character or unit has a special rule that allows them to do so.