Well, for PLA I’m leveling the bed so that in the zero z-axis position, the nozzle is touching the build plate. The first layer height would be somewhat above that, depending on the slice settings. It will be interesting to see if you can reliably measure the difference in nozzle height between PLA temperature and ABS temperature. I think that number will be quite small, but let me know if you can quantify it.
Yes, I also think that is odd to have adhesion problems between the 1st and 2nd layers. I don’t know what to make of that. Does your slicing profile have any drastic changes in temperature, extrusion speed, or fan speed between those layers? It sounds like you might be pulling the part up to detach it – do you have anything you can get between the part and the build plate to start working a corner or edge loose, to lift from beneath the part? I’ve been using a curved-blade exacto knife to get corners started. This is much easier now that I’m printing directly on the glass and I don’t have to worry about gouging the PET.
Are you building the extruder mount pieces with PLA? My personal feeling is that I would want to do those parts in ABS, because I’d be afraid that the PLA might soften and deform, being that close to a hot build plate and extruder. ABS will provide another 40 C or so of margin there. But when you can make your own parts, it’s easy to try different things. Maybe it will work just fine.
But let me know how the dual extruder goes. That’s on my to-do list, along with too many other things.