So, they exiled Arachnus Web with Oblivion Ring, and you're wondering what happens to it later, after Oblivion Ring leaves the battlefield?
When an Aura is entering the battlefield without being cast, you simply choose a permanent that it can legally enchant (this does not count as targeting, so it gets around hexproof/shroud), and the Aura becomes attached to that permanent.
So, you just choose a creature that you want Arachnus Web to enchant, and it is attached to that creature.
If a creature you control dies, then both Butcher of Malakir and Grave Pact will trigger at the same time. You can stack the triggers in any order you want, but it largely doesn't matter here since both triggered abilities essentially do the same thing. This results in each of your opponents having to sacrifice two creatures.
In your scenario, the enchantment being returned is not cast as a spell, so Negate can't be used. Remember, spells only exist on the stack -- if something is going directly from the graveyard to the battlefield, there's no point in there where it's considered a spell.
Some spells/abilities refer to casting a spell in your graveyard; the wording is important, so read the particular spell/ability in question to determine whether the card is being cast, or merely placed on the battlefield.
In addition, if control of a permanent changes, it has *not* changed zones. It is on the battlefield the whole time. The important thing to note is that there aren’t separate “sides” to the battlefield. It’s just one zone, with permanents controlled by various players.
No. Realmwright has you choose a basic land type: Plains, Island, Swamp, Mountain, or Forest. “Snow” is a supertype, and is not something you can choose with Realmwright’s replacement effect.
If your opponent waits until you have already declared Etali as an attacker, then its triggered ability is on the stack by the time they next get a chance to respond. Even if they remove Etali from the battlefield in response, its ability will still resolve, as activated/triggered abilities exist independently from their source once they're on the stack.
So, if your opponent wants to keep your Etali from getting its trigger, they need to get rid of it before the Declare Attackers Step.
Yorvo enters the battlefield with the +1/+1 counters, which means it enters as a 4/4 creature. So, yes, Experiment One’s evolve ability would trigger in this case.
Also, in case it wasn’t clear, blocking with your creatures isn’t considered “preventing” combat damage for the purposes of this ability. The ability is referring to cards like Fog or Holy Day that specifically mention that they prevent combat damage; Questing Beast is able to render such cards useless. Also, a creature having protection won’t be able to prevent damage as normal, either. For example, a creature with protection from green would still be destroyed if it blocks Questing Beast, as the Beast’s Combat Damage can’t be prevented, which is what protection would normally try to do. This is also because Questing Beast has deathtouch, so any amount of damage that it deals is considered lethal.
When an Aura is entering the battlefield without being cast, you simply choose a permanent that it can legally enchant (this does not count as targeting, so it gets around hexproof/shroud), and the Aura becomes attached to that permanent.
So, you just choose a creature that you want Arachnus Web to enchant, and it is attached to that creature.
Some spells/abilities refer to casting a spell in your graveyard; the wording is important, so read the particular spell/ability in question to determine whether the card is being cast, or merely placed on the battlefield.
So, if your opponent wants to keep your Etali from getting its trigger, they need to get rid of it before the Declare Attackers Step.
As for sideboards in Brawl, I don’t think they are used, as far as I know. I haven’t read anything about them, but I could be wrong.