No, even monsters only get one action per turn, and actions should not be confused with attacks. in the case of monsters that can attack multiple times, that's because Multiattack is one action, that just happens to result in attacking more than once. Each attack is not an attack action! Multiattack is also very specific about which attacks the action results in.
So an owlbear can indeed grapple or shove, but that grapple or shove is done by taking a normal Attack action, not a Multiattack action. The Attack action is general purpose and more flexible than Multiattack, but doesn't allow the owlbear to make more than one attack during the action.
Yes, if the second Goblin is not within the Reach of the thing making the opportunity attack
It seems a bit weird, however the criteria for triggering an opportunity attack are:
- You are moving out of the reach of a hostile creature
- This movement is from one of your Actions, Bonus Actions, Movement or Reaction
- You have not taken the Disengage action
- You are not Teleporting
For this question we can discount Point 4, as the Goblin Boss can't teleport.
We'll also ignore Disengaging for now.
For Point 2, the movement is by the Goblin Boss' Reaction, which fulfills that criteria
For Point 1, if the swapping with the second Goblin does not cause the Goblin Boss to leave the Reach of any hostile creature, it will not trigger an Opportunity attack. In the diagrams below, the first set of movement will not trigger an opportunity attack, while the second set will.
EG -> EB
B G
EBG -> EGB
E - Enemy;
G - Goblin;
B - Goblin Boss
Note that wherever the second Goblin is moved from, it does not trigger an Opportunity attack from anything as its movement is forced by the Goblin Boss.
As for using Disengage (via Nimble Escape), it only stops OAs for the rest of your Turn, not the entire Round.
So, if someone had Readied an action to hit the Goblin Boss when the Boss did a specific thing, then the Goblin Boss did that thing on the Boss' turn, and in Reaction the Boss used Redirect Attack, that would not trigger an OA, wherever it moved, as it is still the Boss' Turn so Disengage is still in effect. But normally, Disengage would not help for this.
Best Answer
No, an opportunity attack is a single melee attack.
From the Player's Basic Rules, page 74:
Additionally, multiattack specifies that it can't be used in this way. From the DM Basic Rules, page 6:
Multiattack can be used only on the creature's turn, and it explicitly can't be used with opportunity attacks.
Finally, if you look at the statblock of a creature that has Multiattack, you can see that it is listed under the Actions section of the statblock. Multiattack is a specific action that a creature can take, just like a player can take the Attack action or the Use An Object action. Just like a Fighter's Extra Attack, you can't use it every time you make an attack. A Fighter can only use Extra Attack when they take the Attack action, and a creature can only use Multiattack when it takes the Multiattack action.