OMG sorry, i totally stuffed up that explanation. it was before coffe. errr yer my explanation was poor poor poor and totally wrong! lol.
I tihnk you cleared it up in your second paragraph. Slingshot is on the skill that you training and is affected BY the other skills.
with regards to the first point, I am however pretty sure that slingshot is only within the position classification, tied with the potential capping values
ie:
If you have a PG, with say, Marvelous JS/JR/OD/PS etc and he has Atrocious SB.... and you train SB - the slingshot effect will zero (maybe not zero, but definitely really really small that you wouldn't notice )
Conversely if the above guard had Atrocious JR, and you trained OUtside Shot, you would get slingshot.
Likewise training JS on a C or similar.
Thats how i remember it being discovered many seasons ago - maybe im wrong?