Banishing Light is the "fixed" Oblivion Ring. It got fixed, because many players felt cheated by the O-Ring trick, exiling their stuff for good when it should be only until the enchantment is removed. Uninuitive for many, so Wizards dumbed down the effect to be more in line with what less rules savvy players expected. Kind of angering the rules savvy players in the process.

Adding new rules for new cards that can do the same but different cuz of 1 sentence wording and 2 sentence wording for the same ability.

Sounds very stupid - but wait it is.... but it sells....

No, nothing is exiled if the enchantment is gone before the trigger resolves. This is not an Oblivion Ring type, that has two abilities, one to exile and one to return. This is a single effect with a duration. If the duration ends before the effect begins, it does not begin. There is no trigger to return anything, the exiled card simply returns IMMEDIATELY when the enchantment leaves the battlefield, even if that happens during the resolution of a spell or ability. (So for example, Carniverous Canopy destroying the Banishing Light, allows for a proliferating the Botanical Brawler it had exiled.)

So the etb trigger have to resolve, (if target is still valid) and remain exiled forever.

No, that's an old rule, as old as the card itself, the first printing having been in Journey Into Nyx in 2014 (610.3b, added the previous two rules for context)

610.3. Some one-shot effects cause an object to change zones “until” a specified event occurs. A second
one-shot effect is created immediately after the specified event. This second one-shot effect returns
the object to its previous zone.

610.3a If a resolving spell or activated ability creates the initial one-shot effect that causes the
object to change zones, and the specified event has already occurred before that one-shot effect
would occur but after that spell or ability was put onto the stack, the object doesn’t move.

610.3b If a resolving triggered ability creates the initial one-shot effect that causes the object to
change zones, and the specified event has already occurred before that one-shot effect would
occur but after that ability triggered, the object doesn’t move.