My two cents: take the kids home and stick to the schedule. If you want the extra day ask your wife if she'd mind having D6 out of school for the day. If she says no, listen. Now, if she objects to you taking the kids home, politely but firmly reiterate that no accommodation is available and that you will have them back with your wife at the usual (or agreed, if she doesn't mind the extra day) time.
If they refuse to let you see the kids you're off to the lawyers on Monday.
AND DON'T TALK ABOUT THE PARENTING PLAN IN PERSON! Leave it as an email communication for now. It gives you time to think, an opportunity to bring things here or to your lawyers, there's less chance of emotion getting in the way and saying something stupid and you have written evidence of what has been said. DON'T MENTION IT AND DON'T GET DRAWN INTO A CONVERSATION ABOUT IT.
Me: 31, W: 29 T: 4 M: 2 Kids: 3 (SS: 7, SD: 4, D: 3) Separated, still living together: Nov 2013 Separate bedrooms: Feb 2014 W working away; kids with me: Nov 2014