I think a good rule for balance suggestions is this:
"If you can't personally mod it in and you haven't met anyone who has, don't suggest it."
The opening post is a great idea but it isn't a balance suggestion. It would require Relic to spend significant time and effort on something that is unlikely to materially affect their sales, especially when emplacements have traditionally been more popular among less-skilled and new players.
The problem with not understanding how to mod something in is that you don't necessarily know how the game works. All of your suggestions are fairly easy to implement (regardless of their actual balance) but Counter Battery can only target one unit at a time already.
This might actually be possible if you made emplacements decrewable like USF vehicles. The UKF does have a vehicle squad in the files and I have tried but I haven't been able to get it to work yet.
Retreat prevents your unit from attacking or being controlled and provides it with both a damage reduction and a speed buff to help it survive until it can be reinforced. The exact duration of Retreat can be adjusted with forward retreat points.
Brace prevents your unit from attacking or being controlled and provides it with both a damage reduction and a slight armour bonus to help it survive until it can be reinforced by other units. The duration of Brace is a set thirty seconds.
While the exact balance of Brace and emplacements can be debated, Brace itself fits with the design of the game.
The problem with de-crewing is that pop cap and man power rates can be abused then, like with the USF vehicles and their crews. However the idea sounds interesting in the modding perspective, I've even thought about it myself but decided to go with the less hassle option and just give the emplacements detonation abilities like the previous sWS HQ trucks. Another cool idea was towing of guns like what eliw00d showed in one of his videos but yeah...
All in all modding is what I took up so I can prove my balance point, but with all of these updates and stuff I've lacked both time and motivation since almost every mod on the workshop is broken after an update, or at least some part of it.
It could be done however, that's why I believe modders know best how to fix and balance something, since they know how the game and engine work, all that is needed is play-testing, time and knowledge, that's all, we're basically able to do Relic's job here, sadly, not getting paid for it.