Clarify what you mean by "player D mucks". Dealer should hold on to player Ds cards until player A shows his hand.
If Ds hand is in the muck (which it shouldn't be) then A can have it without showing as far as I'm concerned as he's the only player eligble for that pot with a hand. But it shouldn't come to this with proper dealing.
Why? If player D mucks, the dealer should be immediately mucking his hand and never holding on to it. Its dead and cannot claim the pot.
Player D can't "muck". He can surrender his hand at showdown. This is not the same as mucking. Assuming Player D was folding in turn then player A can either:
a) show his hand and win the pot
or
b) request to see player Ds hand, at which point Ds hand is live again. pretty douchey thing to do. just show your hand.
Player D shoved the flop and its his showdown for the sidepot. He has the option to table his hand to claim 1 or both pots or muck and surrender his hand. When he mucks his hand is completely dead and player A wins the sidepot by default.
This is incorrect. His hand is in probability going to be dead. However, if Player A requests to see the hand then it is NOT dead. If player A tables his hand then it will be. Because it is at this point not dead we're still at showdown. So Player A still needs to show. If Player A for whatever reason refuses to show then the dealer should show the hands in order, in which case they're both live. At any rate: if player A wants this pot then his cards are going to be shown.
This is the rules for MOST places.
I recall the UKIPT allowing you to take a pot without showing. I don't particulary agree with that rule, but it's what you're describing.