Tough questions to answer as a system is a sum of all it’s parts.
1: The easy way make a separate animation blueprint for each of the weapons types you use. It adds more clips but as a resource hit it’s next to nothing. If you want to get fancy then your into a procedural driven design that makes things more difficult to update content side and a lot more difficult to make work properly. Best plan keep it simple.
2: Avoided the x-ray effect and keep the player out of the wall. Not sure if it’s the case in UE4 but typically the ray trace for a shot is projected at eye level and not from the muzzle of the weapon. (Would like to hear about this myself) but in general player clipping = bad.
3: Yes. In UE4 you do not use the mesh player model directly but build it up using a Character blueprint that controls all aspects of the player model and can include everything you need as Components. You can build the character using separate part of the complete character. Things like arms, legs, and even heads. Think of it as one stop shopping and the Character blueprint should be your first stop.
Using a 1st and 3rd mesh still makes things easier but simple enough to import the components and still share the same rig.
4: Good question but there is no reason you can’t do both. At its core it’s just a different camera perspective you can offer as a choice and the only real question is how much time and effort you want to put into it.