How did you get so many points and not know that?

This is an impossible scenario. The only way that an enemy unit can be located in one of your home centers and be disbanded is during builds phase. This *requires* that the unit occupy the center during the fall phase, which means the enemy unit controls your home center by virtue of occupying it during the fall phase. Even if the unit disbands, the other country still controls that center. You do not own it, therefore you can't build there.

Yes. Even if you disband during the retreats due to a dislodge, you still wouldn't get a build.

Italy takes Munich in the spring. Germany retakes Munich, forcing a retreat. Italy opts to destroy the army instead of retreating to, say, Tyrolia.

Germany might get a build out of the deal if someone else controlled Munich earlier. However, because Germany has now occupied Munich, the build wouldn't go there.

No.

Indeed, centers are taken in fall - after the fall retreat, if any - and you can't build in centers you don't own.

Taking an enemy home SC means they can't build there for at peast two winters, as during the fall turn where they retake the SC it will be occupied by the unit which retakes it.

chluke: I actually find your question rather interesting.

The rules on pages 18–19 are pretty clear: calculations of which Great Power owns which supply center are made after the retreats phase (incorrectly before the retreats phase on webDiplomacy), and leaving a supply center empty does not change ownership. So a Great Power can capture one of your home supply centers in a Fall phase and then disband the unit which occupied this supply center. Since adjustments were made at the end of the retreats phase, this great power would still enter the builds/disbands phase with ownership of your home supply center, impeding you from building in it. You can find the rules here:
http://www.stabbeurfou.org/Regles.php

But this 'issue' you bring up could trip up an adjudicator. Of course Lucas Kruijswijk caught this:
http://web.inter.nl.net/users/L.B.Kruijswijk/#6.I.5
Or did he?

KingCyrus' response is actually my favorite (+1). I pretty much just had a brain cramp.

*bows* thank you