A lot to cover here and tbh I don’t have the time to answer each point individually, so I’ll just make a few general observations.
First, as I said in my OP, I’m not totally against nerfing where necessary. In the case of Boom Blast, I think it is absolutely essential that the minimum cost of deploying a weapon during your turn be increased to 1AP, which would stop a lot of abuses firmly in their tracks. Also, I wouldn’t be averse to making BB allow a soldier to deploy up to 2 explosives for 4 AP, but no more - though I think the first point does much the same more elegantly (until someone finds the loophole 
).
The problem I have with most of the nerfing suggestions I’ve seen so far is that they are punishing players who don’t abuse the skill by removing the things they find cool about it, in order to satisfy players who do abuse the skill but don’t want to.
Personally, I refuse to spam 3 explosives/turn because I believe it’s as ludicrously exploitative as the old Sniper RB, or stacking RC with all the Terminator buffs you can manage. But that doesn’t mean the abuse should be allowed - I certainly do not subscribe to the ‘if you don’t like the abuse, don’t use it,’ school of thought.
Hence my suggestion. No matter how you dress it up, all the suggested nerfs are really there to address the one big elephant in the room - atmo BB allows you to deploy more than 2 explosives
per turn in various different ways, and that basically breaks the game.
I agree that for every evolved immunity there should be some balancing penalty to the Pandas, and I’ve long been on record advocating that. In this case, a movement penalty seems most appropriate - the Pandas lose 1MP for every 20% immunity, something like that - and yes, it probably will take some time to get the balance right, but that doesn’t mean we shouldn’t try.
I also don’t think it should apply to ALL Pandas. In an ideal world, it would only apply to those Pandas that were affected by it, but Snapshot doesn’t have the resources to do that (and sorry @stewpidbear, but no matter how good your suggestions sound, none of them are remotely implementable in the current environment - small steps, my man, small steps). Since abusing BB usually disables Crabbies and Tritons, I’d advocate it being applied to them only: so that if a player routinely takes out 6 or so Crabtrons at the start of every mission, they gradually build up an immunity until they are not rendered completely useless after a first strike. THEN, the evolution stops. At that point, it’s done precisely what the nerfers are asking for, just in a different way to what they are suggesting - because let’s face it, the only real point of nerfing BB is to prevent it from disabling 6+ Crabtrons in the first few turns of any mission. And it has done it without ‘punishing’ players who don’t abuse the skill or by removing the other utilities they find cool about it.
Which brings me on to range. Just because you don’t use the skill for its rangefinding ability doesn’t mean that others don’t find it really useful. BB coupled with a GL is a great way of turning a mid-level heavy who couldn’t hit a barn door at 20 paces into a useful mortar support for the stronger members of the team, without exposing him to too much danger. Take away the range buff, and you take away some of that utility (and adding a 25% sop to that doesn’t really do much, to be frank).
As to the meta: so what? Actually, I’m going to wrap that in with the point about informing players. No, you don’t have to inform players about the exact detail of the rule. XCOM never ever told us that if we researched Gauss weapons, we’d find ourselves facing Mutons. We discovered that through trial and error, and those of us who played the game more than once started seeing the patterns. Then we meta’d those patterns, so that you made damn sure your squad was armoured and equipped to deal with Mutons before you invested in Gauss weaponry. It’s all part of the strategy of the game. As I think @Ementrude put it, part of the challenge is realising that this is going to happen and figuring out how to deal with it.
So the only thing you have to tell players is: ‘If you overuse certain types of weaponry, the Pandas will evolve to counter that.’ No more, no less.
I agree with @MadSkunky that the most aggressive evolutions should be restricted to Hero & Legendary play only.
So I come back to my original point. We have a unique opportunity to try an experiment here. The devs are open to it (tbh, I think they like the idea of a more dynamic system to control some OP abuses), and within certain technical parameters can give us an approximation of what we would like. If it works, great: we have a totally unique game on our hands - and not only that, we have the game that was advertised , which I for one originally signed up for. If it doesn’t, get out the nerf gun and put on your flak helmets - because the problem with all the nerf solutions I’ve seen so far is that they punish the many for the abuses of the few; while I think if ‘punishment’ is the word (and I don’t, I think ‘challenge’ is a far more appropriate word), this system has the opportunity to challenge the few who abuse the skill without punishing the many who don’t.