Yes, all is resolved. However, what I learned may provide information for a Part 5: of your excellent tutorial.
I installed ubuntu according to Part 4: UEFI systems of your tutorial. When I booted into ubuntu my screen resolution was only 768 by ??? (I forget the second number), which is only a medium resolution for my monitor. I have an nvidia graphics card, and when I had installed ubuntu in Legacy mode with GPT my screen resolution was 1280 by 1024.
I wanted the higher screen resolution from my nvidia card and I wanted to use GPT for all the reasons that it is superior to MSDOS. Doing some more research I have come to the following conclusions: Legacy Mode does not require a boot partition, so most people who are using Legacy with GPT are partitioning their harddrive as if it were an MSDOS partition. This is what introduces the instability. It is GPT which requires a boot partition for stability.
I booted to a live USB in Legacy Mode. I partitioned my harddrive as you as explained in Part 4: UEFI systems. I flagged that first partition in gparted as legacy-boot. When installing ubuntu, under Something Else, I mounted that first partition as reserved for bios-grub.
The installer accepted these settings without question. My screen resolution using the nouveau driver is 1280 by 1024 as it should be. All is well.
Oh! I almost forgot. Someone might ask, "Why didn’t you just install the nvidia driver from Additional Drivers. I tried this and it crashed my system. I leaned that there is an inherent conflict between the nvidia drivers and UEFI mode that crashes the system.
Anyway, if you want to use GPT with Legacy mode, or have an nvidia graphics card, I believe this is the correct way to go. I’ll keep you posted on any future instability, but for now I’m keeping my fingers crossed,
Jim