Can bombers drop paratroopers in enemy territories without friendly units attacking from the ground/amphibious assault?
Yes
Do these bonus stack? Ex. A US marine gets +1 in an amphibious assault and there’s an artillery with it so another +1= attacking at 5.
Or is their combat value capped. Ex. The US marines get +1 in an amphibious assault OR an artillery because his combat value is 3(4) in the unit list and not 3(4-5)
Marines do not get +1, they just attack at 3 and have no double casualties. The artillery cannot land until round 2, and then the marines have +1.
But combat value is not capped and bonus do stack–there is no prohibition against this in the rules but it would be rare.