Well I can see why having no instants hurts i.e., no draw spells to draw more counters. BUT shouldn't it be worded, "Any other instants"?
No, the spell would already be on the stack, and not in your hand when the ability is relevant.
Guardian, you're right it should be "instant cards" not "instants". But as for upping the cc to 5, I don't see why it would be necessary. At 4 it is still hard castable in T2 at least. The standard power for counterspells these days is 4cc with a bonus or 3cc with a drawback.
Any ideas for the name? I'm horrible with card names unless I start with it.