The Kokiri are referred to as "forest spirits" as memory states, but I'm not entirely sure. My memory is about as reliable as a calculator without batteries.

Boulder's there most likely as a last defense for recent discoveries of the Kokiri. Also, for all we know, the Kokiri could have found the sword a long time ago and kept it as their greatest treasure. Thus, the somewhat extreme defensive measures surrounding it. Also, it's a "hidden treasure of the Kokiri", so maybe not all Kokiri know about it? Well, that's unlikely since Mido recognized it pretty quickly.

Also, yes, that sword we see at the start of MM is the Kokiri Sword, I recently (a half hour ago) checked. I hate the fact, but it's a fact.

And lastly... they're called K-O-K-I-R-I, not K-O-K-O-R-I. Get it right at least, please...