1) The anime ended it's original run
This should be obvious.

2) “What I’d give to cave in that skull of her’s,” said Whitney.
Another extraneous apostrophe.

3) Funny, I’d have sworn showed up earlier than this.
I think you a word there. Perhaps "he" or "it"?

And this one's more of a "please clarify":
Meta Knight, meanwhile, looked down at said mask fragments, thinking. Backslash made the connection in his mind a bit faster, though, and pulled out a little rod-like device.

What connection are you talking about?

All in all, a good mission, though the two "MOMMY MAY I KEEP IT" lines struck me as a bit OOC. Then again, I'm willing to accept the possibility that I'm amiss in expecting Backslash to act more like Shulk.
But back to basics: a good mission and good expansion on the characters of Whitney and Backslash.

I liked this mission a lot! I do have one question though- what happened to the minis? They were mentioned and then kind of poofed into thin air.

Sword and Blade don't appear in Epic Yarn. However this fanfic writer seemed to forget that Meta Knight appeared in Epic Yarn. Actually Meta Knight in Epic Yarn did help Kirby,after he was freed from Yin-Yarn.

So basically it should be Meta Knight telling Sword and Blade about helping Kirby in Epic Yarn if anyone.