#triggered abilities

Yes, this works and is actually really neat tech.

Dream Halls doesn't care where the spell is being cast from, so you can use its alternate cost to cast cards you discarded via Oskar's ability. This lets you make a nice chain of spells as long as you have the right cards in hand.

Please note that Oskar's ability will resolve before the spell you discarded the card for, so if you do chain this way, you'll end up with a bunch of spells on the stack at once, leaving you vulnerable to cards like Whirlwind Denial.

Magic: the Gathering - Game Error

State triggers, like the one on Emperor Crocodile, are triggers that don't look for an event to happen but instead look for something to be true about the state of the game at any time. (Since such a state can last for a long time, a state trigger can't trigger again if it already has a trigger on the stack waiting to resolve.)

For example, your Emperor Crocodile's ability will trigger during the resolution of Victimize if you sacrifice your only other creature as it resolves. Even though you controlled other creatures both before and after the resolution, there was still a brief period where you didn't and that's long enough to cost you your croc.

The case of Devour is a tricky one, since Devour is a replacement effect that modifies how the permanent enters the battlefield, i.e. it happens during the process of putting the Devour creature onto the battlefield.

Ok, so if you control both of these permanents and a creature ((with some counters on it) and that creature leaves the battlefield, then both the Ozolith and Resourceful Defenses will trigger. Each of those triggers will work fully and you'll end up with a matching set of counters added to both of them.

Unfortunately for you, Elenda is a new object after changing zones and so all the triggers from creatures dying were created by an Elenda that no longer exists and thus won't end up putting any counters on the new Elenda.

Maybe. It depends on whose turn it is, actually. (Yes, the asker specified their opponent's turn, but I'm explaining more generally.)

When Green Slime enters the battlefield, both it and Trespasser's Curse will trigger, but which one will be put on the stack first depends on which of you is the active player, i.e. whose turn it is. That player puts their trigger on the the stack and then the other player does the same.

Whenever a card changes zones, e.g. battlefield to graveyard, it becomes a new game object with no connection to its existence in the previous zone.

Yes, this situation works.

When Cloudshift exiles the creature, Resourceful Defense will trigger. Cloudshift will then return that creature to the battlefield. Finally, the Resourceful Defense trigger is put on the stack and can target the returned creature.