So today I was looking at PSCT rulings and came across something that specifically explains rulings people know about but not exactly why it's that way. I'm just going to share this, because it just amazed me how something so simple is really complex in reality.

Do A, and if you do, do B
-A and B happen simultaneously.
-If A can't be done, B isn't done.
-A is still done even if B can't be done.

When your opponent Summons a monster(s) with 1500 or more ATK: Destroy that monster(s) with 1500 or more ATK, and if you do, banish it instead of sending it to the Graveyard.

The first effect destroying and banishing the monster happens at the same time. If you can't destroy the monster with the effect (such as maestroke and using its effect ) then it won't be destroyed or banished. Now lets say imperial iron wall was up. I use bottomless on a monster, you will destroy it, but it will not be banished. I think thats how it goes. Correct me if I'm wrong.

In "Do A, and if you do, do B", Both A and B happen at the same time. A is required for B, but NOT vice-versa. If A does not happen, then stop. If B cannot happen, you still do A.
^ from Wikia.

So yes the monster just gets destroyed and sent to the graveyard.

As for Maestroke, it still detaches. But I think you already know that. Since it's effect prevented he destruction, however, Maestroke can't be banished.

Also, PSCT is the basics of ruling knowledge. If you don't know PSCT, you can't know the most complex rulings.
And then there's BKSS Rulings, which are a pain in the ass even for the gurus.

There's no BKSS regarding LaDD. It's all about PSCT.