A few questions from a new player

Hello Brothers,

We picked up the game during sales with my brother, we go through each Ops in a specific difficulty, once done we increase by one, rinse and repeat.
At that point we just reached Ruthless, but there is some things we're still unsure about and since searching by myself gave me conflictual informations, I came here to ask

- Is there any changes in difficulty from playing solo compared to a squad of players ?
- Since we reached Ruthless some Tyranids had a red mist above their heads, I could read it's some sort of enraged mecanic but how, when, why ? Do they just spawn among waves or did they enrage because of us ?
- As we got a wave in Inferno Op just before reaching the part where you have generators to protect, we just decided to bail out and get to the door.. then we asked ourselves, does the game "store" a skipped wave and give it us back at some point, or was is a good decision to just not bother and proceed ?

Thank you