i played the first two missions of H3 yesterday, and noticed that the monkey dudes in Sierra 117 were missing, as well as the RvB event in Crow's Nest

anyone else with MCC notice any easter eggs removed or added to the campaigns?

They're in the game. Not only that but there are achievements tied to them. Not sure what's going on with your end though

I was playing co-op with a friend online, it was like that for both of us. we were doing Legendary, if that has any effect on the easter eggs

If I had to guess you looked in the wrong spot.

believe me, ive played enough h3 to know where these easter eggs are supposed to be

Vien and I were playing Halo 2 yesterday and I tried to show him the Grunt Birthday Party and Famine skulls and neither of those were there :/

I wonder if that is some kind of glitch that also happened to you regarding stuff like that.

The Grunt Birthday Party skull in Halo 2 classic Xbox version glitches out sometimes. Its done that to me a few times while playing through that level.

You can't get that skull on any difficulty other than Legendary. The Blind Skull and the Assassin Skull are the only skulls in Halo 2 that can be picked up on any difficulty but the Blind Skull's effects only works, not the Assassin Skull. The Assassin Skull is just there to pick up but it doesn't do anything on any difficulty other than Legendary.