I have a C128DCR with a 1541UII and I am able to run GEOS in 80c mode. However I am not clear on what you mean "to boot on the 128DCR in 80 column-mode".
You need to set the 1541UII in 40c mode and the restart in 80c. You cannot get to the 1541UII menu in 80c.
Try an experiment if you have a chance. Connect two monitors or 1 monitor and 1 TV to the C128DCR. Attach the monitor to the VDC output and the TV to the composite, and boot to normal screen in 80C. Press the middle button of the 1541UII and the menu will appear in the TV screen.
As far as settings I am using :
Note that the REU size can be changed with no impact to the others.
Last Edit: Jun 15, 2017 15:09:31 GMT by nonefornow
The cartridge menus only work from 40col mode. After mounting the disk and exiting the menu, you can then go into 80col mode and use the mounted disk. The REU works fine in either mode. There is a setting for the PHI2 timing, but that is usually changed when the c128 locks up. That PHI2 setting should be enabled -- after that, the timing setting is somewhat specific for each c128, so it seems. 100ns is a good start. I have my PHI2 Recovery set to Off, as well. Seems like the C128 cartridge port is very finicky and not consistent from unit to unit
The latest firmware isn't always on the site, but on the git link supplied above. There, you will find the latest compiled binaries, currently @ 3.1a v498. The unofficial one is built with better c128 support. You can always get the bleeding edge source and build it yourself