Welcome aboard, Captain!
![Kaleun_Salute](images/smilies/Kaleun_Salute-1.gif)
To answer your questions, yes. Getting detected in previous missions will put the opfor on higher alert for the next ones. You can go back and redo prior missions but I don't remember how that goes if you've completed the subsequent ones already. Maybe someone who has played more recently can answer that.
Good hunting!
SilentPrey