It's a cute idea and I don't see a way to plug the holes. I don't find the argument that "the spell would say that" to be convincing. "When the arrow hits" is a perfectly valid condition. But let's say that you overrule arrows that explode when they hit. That's really just kicking the can down the street a little.
Next, make a two-part version, 1) fill a petard shell with gravel, each stone with a blast spell on it that all trigger when the stone marked "boom" you throw later strikes the ground...
I don't think it's possible to patch all the paths for this one through "reasonable interpretation". It's a low-cost, conditional enchantment that has a ton of freedom.
TFT is just very vulnerable to
Mr Welch. Check out the discussion here on
implicit detection magic for more info about abusing TFT magic...