I admit, not an X2 user, but keeping an eye on these discussions for sure...
From what Strummy posted I can see why Cake prefers lanes over layers... layers is sorta kinda these special case clip containers, while lanes are really just like another type of track -- what you can do to a track you can pretty much do to a track lane without a bunch of special case logic and code -- record, mute, solo, automation, fx bin, whatever... The only special case code required is making make the parent track the logical "container track" for the sub-tracks (lanes).
From a functional perspective lanes appear more like an ordered set of sub-tracks, similar to how "layers" are provided in graphical programs like Photoshop, GIMP, whatever. You can hide/show graphical layers, edit and apply FX only to selected layers, etc. But you can also merge layers, which would appear to be the only thing required to consolidate X2 lanes.... I would imagine you want to multi-select some number of lanes, and either "merge up" or "merge down" -- the merge replaces the first(up)/last(down) lane in the multi-select, the merged lanes are removed. Now you simply have a merged lane with multiple clips, like any other track, and consolidated like the old layers. Also, the user has total control over up/down merging, unlike "rebuild layers", which could potentially just do whatever it felt like.
Now the hard part is defining what it means to merge overlapping clips in multiple lanes... or for that matter overlapping controls like automation, etc. I would think sonar first has to determine whether any clips overlap, then give the user the necessary options to deal with overlaps: abort merge, merge only non-overlappin lanes, merge all with crossfade, merge all with prioritized truncate(whatever that is?), manual merge (for each overlap provide subedit screen for direct clip editing at overlap point -- user can slip, crossfade, whatever), etc. etc.