огромное спасибо

Да не за что! :)

Paddington: Where should I stick/shove it? (без UP)

It's a rhetorical question with an obvious answer: 'I should probably stick you guilt up your a**.'

But in the question form, you leave off UP. Otherwise it sounds unnatural.

ok

It’s because of WHERE. However you could say “WHAT do you want me to stick it up?” (because you stick or shove something up SOMETHING, not up SOMEWHERE)

Right, makes sense. Now, suppose, instead of asking 'where I should stick it', I decide to ask a more specific question: "whose a** should I stick it up?" Does it sound natural? (Don't get me wrong, though.. In real life I would never say this!)

Yes, that’s would sound natural because, again, ASS is a noun.

Ok. Super!