@GentlemenGraphic: You're not supposed too. The sword stuff in the code/assets was just me mucking around with a melee weapon. Thought the snippet might come in handy for someone and I may it properly later.
You could use a AnimNodeAimOffset if you had some standard aim offsets and use the AnimNodeSequenceBlendByAim for all the rest.
From memory, Gears of War does this.
Otherwise, yes, 1 frame animations for the AnimNodeSequenceBlendByAim in each pose - CU, CC, CD, LU, LC, LD, RU, RC, RD.
Once created, you turn them into additive animations in the AnimSet editor (see Creating an Additive Animation).
As long as the name of the animations follow the proper naming conventions (Ready, ADD_Ready_CC etc) it will work with out modifying the animation tree further.
You will need idle animations and such too, unless you want the poses blended with the default idle animation
It would depend on how different the animation was.
If it was a simple shift of the left hand on the front of the weapon, then no.
If it was a completely different style of aiming the weapon, then yes.
You could, for instance, share all the animations between an M4 Carbine and a FAMAS, with the FAMAS reload animation in a new animation set.