Spellbane doesn't deflect invoker acid stream. Shouldn't it?

You can spellbane it if it's cast at you.

Which going forward will be the only option.

was there a way to initiate it without casting? I always saw the fact that once it landed you could let it run its course against as villager w/o retriggering spellbane as the most appealing part of the spell.

Vitriolic stream let you pick a target, but it actually always went after the person you were fighting, if you were fighting.

So you could so this:

Be fighting Jastyna
Cast vitriolic stream at the Destructor
Now Jastyna's eating acid without a chance to spellbane, when she would have gotten a chance to spellbane if you cast it directly at her.