Yeah, no amount of soak is gonna save ya from a TD, it looks like. I re-read your post and see your point about it being activated, and I think theres some merit to it. For TD's, maybe they should activate blast for all engaged targets automatically, and activating the blast quality would extend that out to all targets in short (nee' close) range.
Grenades with blast as written I think are fine. You could use those advantages to recover strain or grant boosts/setbacks, but … why? If you're tossing a grenade, aren't you trying to clear a group of enemies? I guess I just don't see where players would reasonably choose that option in situations where they chose to use the grenade in the first place. The alternate TD rule I described up there I think could help avoid that, or add a rule (house or official) that the first 2 advantages rolled on a grenade-like weapon attack must be used to active blast.
And, by the maker, the things got vicious 4 on it… appropriate, but /shudder.
-WJL