Dear Smidge,
I understand the problem totally. Neither of you wants to engage in mudslinging, you've come so far and don't want to sour things now.
But the solicitor is right, there does have to be a divorce at some time. You could of course both enter into a Deed of Agreement as an end-point of the collaborative work, which says that you will seek a divorce in due course based upon 2 years' separation, and will "invite a Court to make an ancillary relief Order in the terms following". And then set out an intention to go for a pension sharing order.
It's not legally enforceable until there is an ancillary relief order, and
Decree Absolute. The Court doesn't acquire power to make the Order until decree nisi at least. So there will have to be a divorce i.d.c.
But if you had any kind of a normal marriage there will have been times when one of you shouted at the other, sulked, got the hump, whatever. It's only down to a bit of "selective editing" or "creative writing" to turn that into "unreasonable" behaviour. If done by agreement, I can't see a real problem.
The District Judge reviewing the Petition will know it's been prepared as part of the collaborative process and will say "ok, I understand what is happening" and nod it through, I think you'll find. They are *very* supportive of collaborative family law.
I know it sounds cynical, but if you have agreed to a tidy end to the marriage and come this far, it's only a bit of pragmatism: "Ok, you allege this and this, I'll say it's all out of context but I won't defend, and we'll get our divorce".
So it shouldn't be an obstacle. The solicitors will help you draft something - it's what they're there for.
Good luck!
Will C