Oh, I'm glad you found someone!
I've been trying to dig up the emails that we wrote back and forth spelling out the details of our plan, but I can't find it.
I will briefly say that our problems were really rooted in Pip's anxiety. A combination of generalized anxiety, resource guarding me, and getting very aroused and disturbed by crazy Squash/Maisy play. With a dash of bratty teenager. So most of our behavioral modification centered around counter-conditioning to the youngsters playing, with a heavy dose of being careful about accidentally socially facilitating BOTH of the boys and removing myself from situations where Pip was resource guarding me. Also a lot of training sessions where they traded off down-stays and working with me (nothing in particular, just tricks or polishing rally skills), and a lot of mutual fun things like walks with my husband and I.
Inside the house, I usually still gate them apart from each other (fortunately due to the layout of our house we can do so without completely isolating anyone) because it is a small space with a lot of corners for everyone to get piled up and trapped in. But out in the yard and on walks everyone is getting along very well at this point and the boys even hang out and play with each other.