Can you reject Slingshot DLC mission first month and choose it next month?
Solution 1:
This information is so hard to come by so I'm posting here in response. The first quest comes very early in the game. I rejected the quest, and it never came back even after my whole squad is on end game armor.
Started another game, and this time I accepted and finished the first mission. And then for the 2nd and 3rd quest, these come ridiculously early as well and you probably won't beat it with your newbie squad with stick weapons, so skip them and do it later. The council SPAMS you with the quest, but there's no penalty whatsoever for not completing the quest when it shows up.
In short, don't skip the first mission, the following ones can be skipped.
Solution 2:
It is possible to accept the Slingshot mission arc but put off doing the first mission. After the council approaches you, select Slingshot (as opposed to Normal/random) then cancel out of the squad view back to the globe. The mission will remain available for a day or two before disappearing. Within a week or so, the council will approach you again asking you to do the same mission. There is no penalty for denying them, giving you some time to boost your squad members
Solution 3:
In my experience the first mission reappeared again on normal. I did it the second time it appeared.