Is there a way to see which missions are locked behind a set date? I tried checking the data viewer but couldnt see it mentioned, but a few events did mention needing specific months like the Golden Academy.
You have to look manually at the ruleset files. The missionScripts file is the most important one, but there may be something of interest in eventScripts file as well.
Checking the boxes I didnt see it from Osiron but did see it from Land Survey but its less than 1%, but might try to get some from cycling my other boxes for the sake of it, as would be fun to try out EMP as never did that damage type before, or bio, or chemical, or anti-E115. I mostly just stuck to Kinetic, Laser, Plasma and Cutting with electric / stun. I was never sure if EMP would be worth it or not since the robots have some resistance to it, but I guess the ignoring armour mentioned in the description helps bypass that?
Just looking at it can be deceptive. For example MIB Heavy Trooper takes 50 % damage from EMP which looks really bad at first sight. But for example EMP Mine the armor effectiveness is 0,25, it has damage 100, and it has 400% stun damage. One EMP mine will stun (sometimes also overstun, leading to its death eventually) usually take down any enemy unit which takes even a little damage from EMP. Within MIB Stealth Bombers if you are not susceptible to EMP yourself, you could just go next to one, drop the mine and move to trigger it, and watch the MIB heavy go down. No need necessarily to even throw it and wait for the end of turn.
CHOKE (e.g. gas grenades and mines) is another extremely effective damage type, one that can be overlooked if you don't look closely.