This is a sensible proposal, however it does take a lot work to implement. Maybe at some time. Also, when will they switch to new weapons? Time switch is punishing; research switch is unfair; score switch is impossible atm. (and possibly also unfair, as the player will have an incentive to play worse and lower his score on purpose). In the greater scope of things, this will be affected in the future by more mid- and end- game sub-factions replacing the current ones as the game progresses.
Amounts of poison gas grenades have been bumped recently anyway. But low tier enemies cannot be given good grenades, because they're low tier. Like I said, making them throw more and always target softer targets would make the AI too predictable - which would make it easier to protect your 'champions' (aka 'valued soldiers') by sacrificing human shields.
Plate mail is, naturally, too good for the difficulty of getting it, but that's a current balance issue - it will be fixed as new elements are added into the mix. Eg. I'll add a gem requirement to it once there will be more sources of gems than just the Mansion.
As for your other problems - (sometimes) losing soldiers/countries to random chance, if you don't like it, you're playing the wrong game. No foolproof solutions to everything. If you don't like to lose so much (even if the loss is minor), play a game where losing is not an option. XCom teaches that avoiding losses in a war, or expecting everything will go 100% according to plan, is a silly fantasy. I don't intend to change that.
Honestly I don't think plate is too easy to get; the rare earth is pretty damn rare unless there's something I can salvage it from which I haven't noticed (other than Raiders). As of July 2602 I have made 5 suits period having only lost 1 (unfortunate mishap with a berserking mininuke armed Osiron).
Second, I think a combination of time and score (as in total points accumulated, not your last monthly) would be the best triggers. That said, having the enemy's armaments rubber band with your demonstrated ability/threat is actually a pretty good idea. Are your gals a PITA? Arm up. Has the threat passed? Deescalate.
Third, you don't have to give low tier enemies good grenades; even the entry level grenades can do serious damage to armour, or at least menace it.
Fourth, any player who needs to use sacrificial rookies to save their better squaddies is a bad player. Between smoke grenades and indirect fire/smoke sniping, you should never have to do something so desperate, and if you do, it's a form of damage control, when all other options have been exhausted that will see you lose a soldier instead of have an injured (or unharmed depending on the armour) elite.
Finally, getting rid of unfair RNG is not about 'never losing', it's again about
having the game's outcomes respect player actions/agency rather than largely ignore them while still providing a challenge. Plenty of RNG exists already with respect to shooting and the like; the difference is that you can readily compensate and adjust for bad luck here where the same is not true with regards to the likes of Psi/Infiltration.
@ivan: Thanks for that. I thought he was talking about a situation comparable to mine (Academy Cruiser vs 6 Rookies armed with Gauss Muskets) which was not the case. Still a respectable base defense though.