I have a situation where four Tie Fighters are invading a Rebel system where the only Rebel ship present is a Rebel Transport. Is it correct to assume that a Rebel leader can still be assigned to this "battle", even though the Transport has no attack capability? If so, then presumably tactic cards are drawn which could include the Transport doing damage without any roll of the dice... Can the Transport do damage even though it has no attack ability (at least via the dice)? Thanks in advance!

Absolutely. There are cards that inflict damage. In lore those transport ships were occasionally used as fireships and could be modified to have a few weapons. Though remember that during the retreat step you must retreat the transport or it gets destroyed.

if they are alone at the end of the space combat round and there are enemy ships they get destroyed

if they are alone at the end of the space combat round and there are enemy ships they get destroyed

Actually it's if they're alone at the end of a combat round. They get a chance to retreat if they survive the first combat round.

It's different than Structures which are destroyed immediately at the beginning of a combat round if alone.

It's different than Structures which are destroyed immediately at the beginning of a combat round if alone.

Actually, structures are destroyed during the last step of combat round (if alone).

There are no units which are destroyed "at the beginning of a combat round".