2. Killer Blow is really weak, possibly as a way to balance out its stackability. Balancing against it seems slightly unwise to me.
No argument from me; in fact I already pre-agreed with you on that point.
3. Would limit the final version of Blessing Of Heimdallr to one use per action. Otherwise I like it. The part about locking down a die that rolled +1 is clever, I will steal that idea.
It's inherently limited to one use per attack because no aspect can be invoked twice on a single roll, including "the aspect linked to the Sword of Heimdallr". I suppose someone could try taking multiple aspects all referring to the sword, but my intention was that only the aspect that was specifically linked to the IoP (per the IoP 'Musts') counts. That said, I think you could potentially invoke it (and get the bonus) multiple times as part of a sweep attack (once against each target) and this is probably fair.
And I can't claim credit for the cleverness of the mechanic; I stole it from the 'Unshakeable' stunt from YS. But I agree; I like it too.