I think you're right, but it should only include home territories inside the bolder lines. For instance, if Russia was up in Bohemia but had since withdrawn, Austria would not need a declaration of war on Russia by moving into the vacant Bohemia. But if Russia still had an army there, then one would be required.
And furthermore, if Bohemia were still purple, but vacant, and Germany moved into Bohemia, I would say this would have to consitute a declaration of war against Austria, given that Austria still survived.
An alternative to that is that the home non-SC territory has to be of the same color as the country that owns it to assign the gravity of the necessitation of a declaration of war. This means, once Austria "lost" Bohemia to Russia (even though peace was between Russia and Austria now and Bohemia is vacant), Austria would need to sort of "reclaim" that particular home territory for it to be part of its sovereignty, meaning if it left it purple, then Germany could move in and still have peace with Austria, which is to Austria's detriment. Thus, it would be in Austria's best interest to move onto the purple Bohemia and "reclaim" the territory.
Does this make sense, and which option should we take, Option A (the one where any home territory is yours forever unless occupied) or Option B (the one where you have to reclaim, but can only have claims inside your home borders). Option B is my favorite, because I can see Austria being confined to, say, Greece and being totally exiled from Germany, and the Germans would have to go through the whole farce of a declaration of war on Austria just to step on Bohemia. Also everyone should note that this only applies to non-SC areas.
As to ocean squares, I say that any of those are fair game any time. If I'm France and I somehow get a fleet up in Barents Sea, that's where it's gonna be even in a time of peace with Russia. If Russia wants to dislodge it, they've got to declare war first.