Yeah, sounds like heavy just needed to respond to the Local's question by simply saying, "Yes, but we're unable."
It's the first refusal of this type that I've ever heard.
Call your colleagues in LAS about answers to your 4 questions, as visual separation is common practice there. They clear guys for takeoff while the preceeding departure is still on his takeoff roll.
I think flight crews are very willing to help his friendly ATCer get out of a potential bind. Just listen to the responses you get when you query a flight about his speed or climb/descent rates.
"...Mach .79, whaddya need?"
"2000 fpm, whaddya need?"
They don't seem to realize that you're asking out of curiosity and not necessarily because you "need' anything (apologies for the slight thread creep.).