Do you exit sub-object mode before adding the Unwrap modifier? You should. Sub-object selections are passed up the stack, so the upper modifiers are limited to just the selected bits.
Collapse the modifier stack before you edit the Smoothing Groups. Editing them below the UVW Unwrap modifier shouldn't cause the UVs to be reset, but changing the vertex count always does, and you're probably doing this inadvertently somehow.
Yes. If you see a yellow & red highlight, you're in sub-object mode. If that's on, and you go up the stack, you'll see icons on the right side of the upper modifiers, like the 3 plusses here...