The change is simple and easy. Simply revert AOE damage changes, keep the munition reduction make the weapon homing and remove the affect that make the vehicles immobile.
Simply this, simply that. There is nothing simple about this suggestion, as I've already told you. Turning it into a sticky AT satchel for IS would be a huge gameplay concern for obvious reasons and "in the worst case" having it replace the Sappers AT grenade is not a straightforward solution either as it'd force players into getting low range snares which they might not want (amongst other things).
Having it replace the AT sections' AT grenade doesn't make any sense because it's stock tech that would then only do something for a singular doctrinal ability in 1/9 doctrines. I didn't see any proposed alternative to replace it with.
Furthermore I have no idea why you think Gammon Bombs and smoke are such a huge issue in the first place. Almost all British tanks already have access to smoke anyway (so barely anything changes there with the addition of the Pyro smoke barrage) and using them to kill a Schwerer (which is rare anyway) requires
1) end stage tech
2) a boat load of munitions
3) an enemy that isn't defending it
4) massing all your infantry squads in one location which can have consequences for map control
and 5) a possibly dangerous overextention
which seems like a fair risk/cost & reward situation to me.
Gammon Bombs are currently not an issue and there are many more important things to look into with the limited time and resources that are available.