In my case with 5.7L ,, I could rotate the butterfly by hand !
I'm not sure how effective to spray the throttle body cleaner to clean the manifold !! I don't think it would do anything as it will instantly evaporates !!
Is your MY 5.7L drive-by-wire? A mechanical throttle, yes, you can rotate by hand and open the butterfly and clean the whole thing. If you forced the motor driven drive-by-wire throttle body open, I was worried about damage so I gave it a light press and when it wouldn't move, I abandoned that idea.
Yes, the cleaner would evaporate, thats why a drench as much as fast as possible without stalling out the motor. It cleans the opening of the intake manifold for other vehicles I've done this, can't tell for the rest of the manifold.
Eitherway, its arguable if it really does anything to help, a light coat of soot on the throttle body and intake manifold really does NOT effect performance. It is possible that the soot can build up enough to effect performance, and likely it would be engine idle it effects, but it would take a while and you could easily wait until you get symptoms and clean the throttle body then.
It appears the drive-by-wire sets engine idle the old fashioned way, it cracks the butterfly valve open. Most fuel injected, mechanical throttle bodies, linked directly to the gas pedal, seal the butterfly valve shut and use a seperate air circuit with a electro-mechanical valve to control idle. That smaller restricted air circuit is what is susceptible to clogging up somewhat with soot and effecting the idle. With drive by wire and NO seperate idle air circuit, I doubt you would ever get the throttle body dirty enough it would effec the idle. BUT, I'm guessing.
i.e. It can't hurt to clean the throttle body, don't expect it to get you anything. If your going to DIY, and spend 20 minutes and $2 on a can of cleaner, go for it, I don't think its worth the money to pay a mechanic or dealership to clean it, especially how a dealership may charge you $164 to do it.
BTW, its NOT anywhere in the maintenance schedule to clean the throttle body.