Since I've gotten the mechanics to work, I expect I will spend more time with the printer just doing its job while I can do something else, so I took it home over the holidays, it can stand in a corner printing holiday cheer:)
I bought some grey PLA from igo3d.de, 1kg of it. Since it's not the same size spool as the spool holders that came with the Mendel90 kit, I just put a stick across the back plates, as shown here:
I got a nice test case from Thomas Riedl, a small piece for holding wires in a LED tube. I did some modifications to make it more rounded, but otherwise it's the same. I also did an extended version that allows mounting a backplate with M6 bolts. I printed one, and it came out slanted.
Just to check that it wasn't a problem with my redesign, I printed one of the originals. When starting that, despite the pronterface saying the extruder temperature was at 185C, the firmware prevented cold extrusion. Odd. Then I got disconnected.
Turning back on and heating properly. Despite what I thought, it will print from wherever the heads are in X-Y, so I need to always center before starting a print.
The initial outline has trouble sticking. I suspect I need to lower the Z target a smidgeon.
In the second print, I don't see the same skewing until at the very top, but the layers are sloppy. I probably need to adjust somewhere what the extruder diameter is. Simplify3D's troubleshooting guide has a nice illustration of what's wrong when prints look like crap in various ways, I'll let that guide me.
So three apparent issues:
Poor attachment at the beginning - adjust start height
Wrong extrusion thickness setting - find the place in Pronterface that sets this
Skewing - unsure, possibly from the motors being right at the edge of getting enough power.
Here's Thomas' print and my first two attempts at doing the same: