Sorry that's a lot of reading. Got bored.

This is a very interesting and well-though build and I can't wait to try it out myself. But I have a few suggestions base on my personal opinion to this defensive build to make Koshka a slightly harder to deal against:

1. I would say crucible or atlas are better defensive items for Koshka. Crucible protect can protect your teammates in a team fight and atlas can slow down the enemies in addition of frostburn making escaping impossible. The only time where metal jacket is favorable is when all of the opponent's hero are heavily pure WP(no bonesaw or piercing spear).

2. I highly recommend no more than 1 fountain of renewal. The reason for this is because no matter what defense item you put on Koshka, she is still squishy and easily killed. Two fountain will largely limit Koshka's offensive output and don't really sync with her other items. Because think about it, if you are killed under the 3 second heal time, then a second fountain can't do your allies good anyways.

3. A crucial point about Koshka's burst crystal damage is that it can pinch enemies down and grant critical basic attack. But to do this, she will have to use her skills under a ver short time to maximize the damage. So I think, instead of a second fountain like I mentioned above, you should build a clockwork to drastically lower your cooldown acceleration.

I'm gonna have to try it out. Thanks!