I don't know about your own troops providing cover for the enemy. Is that in the RB? I at least cannot recall this.
You cannot shoot into melee, right, but you can shoot someone next to a melee if he's not involved in it and you can certainly shoot someone behind a melee if you can see him. In an elevated position for example.
It's right that the ratmen shot at the marauder henchman if he was the closest target, they were not in an elevated position and there was no Large Target around.
Hitting on 7s is not specified in the MH-RB I believe. In the 5th Edition Warhammer RB however, hitting on 7s ment you needed a 6 followed by 4+
Hitting on 8s ment 6 followed by 5+
Hitting on 9s ment 6 followed by a six
Hitting on 10s was impossible.
In MH unlike Warhammer, there is only cover and no cover, nothing like partial or full cover. If you are in cover, the enemy just gets -1 to hit with a ranged weapon.