Hey, it is a game that you play for fun, if you like saving/loading, this is entirely up to you, do not ever let anyone tell you how to have fun lol.
And yes, there will be a pop-up at the beginning of the mission informing you of the presence of an alien officer. November is not that late, so no worries. It also depends on what difficulty you are playing (the lower, the slower game progression due to "invasion ticker" multiplayer,which gives you more time to develop).
@ralph_09 Initially it is difficult to stun Sebillians without gas, you may try stun batons, but personally I think it is too dangerous to try and not worth the effort (they are quite weak). Wait for gas grenades or you may try another trick - when you enter the UFO and you see enemies inside standing by the power sources, try to shoot/grenade at the power source instead of the enemy - they explode both with incendiary (which kills) damage and stun damage - incendiary goes first and the stun follows, which quite often incapacitates the enemy (mostly Sebillians, Ceasans are a bit more fragile and can die, but it is still possible to use that against them).
And just to clarify - I am not an expert on this game, but ask away if you need any help.