Cat I hear you on the OW/step mother issue. I've been through similar issues myself but OT is right you have got to let it go. When I first came her my focus was solely on OW and what she was doing to my family. She did meet H after he left (although it was cutting it very fine!) and so I have to accept that she didn't cause the breakdown of my M. She certainly hasn't helped what has gone on since and I have hard evidence to prove that she has indeed made sure that things have got pretty heated at times. She once told my S16s GF that she felt guilty about splitting up our family BUT clearly not that guilty to walk away and find someone who was more free to be in a R.
Let the OW go today (and anyone like her that might appear in the future to also get an equally large engagement ring - I've been there too with Hs OW). She is not worthy of your thoughts. IF she steps out of line completely with your kids THEY will tell you and THEN you can take appropriate action BUT bossing them about is not one of those things that needs drastic action. She is a mum herself and what you don't know is if this is simply the way she has learnt to parent (don't get me wrong I'm not defending her just looking at it from a different angle). Let's face it I'm sure there have been times when all our children have complained about us 'bossing them about' I know my children certainly have but it doesn't make us bad people. In my case the one thing I WON'T stand is when OW raises her voice to my kids and I have tackled both her and H on this and as far as I know it hasn't happened again.
On the counselling issue. Does S10 want to go to counselling? If he doesn't you are on a hiding to nothing as there will be no achievement made (just like when you went to MC and your H didn't really want to be there). You can be his confidante, you can be the one to give him all the love he needs, you may just be enough to calm the storm that is obviously raging in his little head right now.
Take care.
Me 43 XH 45 M 2.7.88 Divorce 7.10.09 Kids D20,S17 & D15