Allies generally don't give partial cover. 5E doesn't base cover on squares like previous editions, it's based on the creatures themselves. While it's reasonable that a halfling could be hiding behind an ally as they are working together, it isn't really reasonable that someone being in a 5 foot cube beside an enemy in a 5 foot cube is going to have half their body being obstructed. The only time I'd rule there could be cover is if all three characters are in a straight line. If there is any angle at all then it is almost geometrically impossible for there to be cover unless there are some size differences going on.
Yes and no.
Under Cover PHB pg. 196
Yes - "A target can benefit from cover only when an attack or other effect originates on the opposite side of the cover."
So yes you can move off to the side and shoot if that angle is available to you.
No - "A target with half cover has a +2 bonus to AC and Dexterity saving throws. A target has half cover if an obstacle blocks at least half of its body. The obstacle might be a low wall. a large piece of furniture, a narrow tree trunk, or a creature, whether that creature is an enemy or a friend."
Creatures both allies and enemies grant cover.