Question 1: Best practices for animation variations:
Weapons as a prop more important than animations is the need for function over form. How many rounds, what kind of damage it produces, sound effects, and so on.
Did a behind the scenes video of how we are setting up our weapons BP.
Every time I need an addition as to function it’s added to parent class and the required function is passed to the character who has possession of the prop including animation. Whatever the player needs as far as animation goes comes from the weapon being used and not unique to the character model.
Question 2: Blendspaces:
Blendspaces is nothing more than either a 1d or 2d index that contains a animation clip at a given address so more ideal for locomotion and not so much for action states. Overall montages seems to work the best as far as to how animations for this purpose should be packaged. You can change the “scale” of the animation once it’s in Unreal 4 but forcing a 2x speed just makes the animation looks like it’s playing back at 2x speed.
P.S. It’s not called re-authoring but iterating. 
Question 3: Best Practices:
Well as to authoring of animations with out context as to the output of the work, where it’s going so to speak, there is no real best practice as compared to traditional work flow habits.
There are just to many considerations of just making things move as compared to work that needs to be of acting quality and the fail of Mass Effect in this area makes for a fine example of how bad things can get when the talent of the artist is overlooked in favor of cheaper procedural driven animations.
Playing around with frame rates though this is an interesting opinion.
Some bullet points.
Being an animator it’s about being able to look at something and understanding why it looks right even if it’s not. The closer you come to falling into the uncanny valley the stranger it looks and the harder you have to work. Stylized though you can get away with murder.
It’s not the state your in that mess things up but how you got into and how your getting out of where you are that makes the difference between something that looks smooth or not.
If working with in place you will be fighting with the tendency of the engine wanting to move at constant rates so to make something look good at times you will have to animate bad. Just try making a ladder climb or a jump in place look good.
Pulling down from 120fps to 30fps is like over compressing to mp3 to hit a target file size. All your doing is sucking the fidelity out of the movement and still have to clean up the mess like any other kind of mo cap data.
Keep in mind that it’s possible to add unnecessary actions once the player has made a decision that forces the action animation to look sluggish. Going from the hip fire position to the iron site fire position is a big jump when the player has already pulled the trigger. Sometimes you have to pop the animation based on the demands of the game.
Last keep in mind that your animating for a game engine and not an audience so in all cases the engine wins the argument of how to do it right even when it’s wrong. Kind of like being married. 