Help with blendspaces and their uses

I posted a similar question in a few other places, but I realized I might just not understand blendspaces and what I can do or not do with them. To show my issue, I made some neat little pictures so this doesn’t become a wall of words.

Currently, I have blendspace already in and working. I use them for simple things like blending between walk and run and having the character look toward the cursor, like so:
problem illustration just head.png

Okay, very easy, very simple. When a character has a melee weapon, they look only with their head. Weapons that are fired are aimed normally. Both work fine. Now, some melee weapons can also launch or fire things, like having a knife and also being able to throw it. I have a blendspace for the throwing animation, as well, and can play just that fine, like so:
problem illustration just throw.png

The problem is, playing the animation plays it across the entire skeleton. I’d like to blend it with the lower body, like so:
problem illustration ideal.png

I noticed additive settings in the blendspace, but doing anything with them or adding a base pose contorts the character into a big mess. In code and in BP, I can’t find anything to let me blend with blendspaces outside of the animgraph. Slots also won’t work outside of anim sequences or montages. I’d thought about extracting the pose from the blend as an anim sequence, then playing it in a slot, but haven’t had much luck, yet.

Is the only proper way to handle this by always having a blend or offset to face a direction, then play a montage over top of the blend? Any help with the problem or explanation to clear up how/if I’m misusing blendspaces would be greatly appreciated.