First of all, I can't believe your ped told you that cosleeping ruined her marriage, that is unbelievable to me. I can gaurantee that there were other issues that lead to the demise of her relationship, don't let her scare you.
Second of all, it sounds like your current arrangement isn't working very well and I don't blame you at all for trying to find other solutions. I have a couple of suggestions from my 7 yrs of cosleeping w/3 kids.
I have always put a toddler bed in between the wall and our bed, at exactly the same level so lo can still nurse and lay right next to me. It works really well bc the crazy sleeper has their own space and is not kicking daddy at all at night (and I have some extra rm too).
Also, I have noticed w/my boys that if they aren't exhausted by bedtime they probably need a change in the nap schedule. For example, the last nap is too late or a nap needs to be dropped sometime during the day.
Parenting is full of adjustments, compromises, balancing the needs of the children w/the needs of the parents. Everyone in the house is important and everyone needs a good night's sleep.
I am sure that there will be other posters who will have good ideas too. Try different things out and see what works for you. If you and your bf work this out together I am sure that your relationship will be even stronger. GL!
Wife to dh, Mommy to ds1 12/2002, ds2 9/2005, and ds3 9/2008.