Okay... So it's not just the "Si is around all the time" and "I can't do the poly" thing, then, that brought you to your breaking point? It's the "my wife and I have discussed and agreed upon what was best for our children and now Si has put that on hold" thing as well?
Honestly, I can better understand your "butt out" mentality on your second point rather than your first. Seems to me, then, that the move had been discussed with everyone, that you and FoL made decisions with respect to your kids, and Si balked against that decision, tossing your actual plans up in the air. Do you feel like you have been undermined as a parent (or as a partner in this marriage in general?) by someone you don't feel is a co-parent or co-partner?
Yes, please do continue the remote counseling and continue to try to get at the root of your anger - if one of the roots is feeling undermined - that any decisions you made can be undercut by a word and not supported - then maybe you should work on that to start?