My enemy has a 5 might unit on a battlefield and combat starts, I attack and my enemy makes my unit move back with a spell. Is this a combat win for them? Similarly, they star cross it, is this a combat win? Similarly again, I get stunned, I cannot conquer and recall, this is a tie, correct?

Partly — they win the combat if only your unit leaves, but not if Star-Crossed clears the battlefield entirely; and a stun only makes it a tie if your unit survives the 5 damage.

Moved/returned to hand: Yes, that's a combat win for them. Once your attacker is gone, the Combat Damage step simply doesn't happen (no opposing units), but the combat still resolves in the Resolution Step, and 468.3.a gives the win to the player who is the only one with units there — the defender. So "when you win a combat" abilities trigger. They don't score a Conquer point though, since they already controlled that battlefield.

Star-Crossed: No. Star-Crossed returns a friendly unit and an enemy unit to hand, so their 5-might unit leaves too. With no units at the battlefield, the result is "No Result" (468.3.d — neither player has units present), so nobody wins the combat, and the battlefield actually becomes Uncontrolled (468.5.b). No conquer point, and no combat-win trigger.

Stunned attacker: It depends on your unit's might. The stun means it deals 0 damage while the defender assigns 5 to it — if your unit has 5 or less might it dies in the damage step, leaving them as the only player with units, so they win the combat. If your unit has 6+ might it survives, gets healed and then recalled during Combat Cleanup, and that recall forces "No Result" — a tie, no combat win for either side, and they keep the battlefield.