It would be really easy to make a treaty covering the debris. The most horrible one would just require multilateral signoff on every removal action, but that would work just fine, every one is OK with clearing it out.
Depends. Denying US access to any tech hurts them more than the armies that don't have such high tech capabilities. On the other side - if you US are mostly unaffected by that (preparations etc) they will be so far ahead that cleaning up the debris will hurt US superiority.
So - who will be ok with cleanup in that current moment can vary a lot.
To see a lot less serious mess unfolding in real time - Syria. While the end game is clear for all - get rid of ISIS and install strong stable government, everyone is moving to get advantage and increase its bargaining pool.
Everyone with space launch capabilities would be fine with it, and they are also the relevant parties when it comes to treaty issues. I don't think Russia and China would want to leave junk up there, I don't think any one else is even questionable.
I regret to say that when it comes to international negotiations, just because everybody is "OK" with something does not mean someone won't see an opportunity to make hay out of appearing to have a problem with it. They can hold out for money, use it as an opportunity to burnish nationalist credentials at home, any number of things.
What if your junk includes proprietary or classified technology? What if your nation state at one point put something into orbit that violated a treaty?
I'm sure there are a few reasons that relevant parties might not be totally cool with rival nations pulling their debris from orbit.
And when you refuse to clean a piece of "debris", everyone will know it's your spy-sat, so you (and everyone else) have all the incentives to deny cleaning things at random.
