I suppose that the chart is not literal. I believe GMK uses ISO row numbering, not SP row numbering.
No need for ANSI Enter.
Is the home-row Control key stepped or centre-stemmed?
I suppose, I am only familiar with this form of row numbering, seems to be the most common (even used in GMK renders afaik)
ANSI is only supported through left shift, enter, and pipe so as to enable people who may prefer ANSI but still use their own layout or just plain ANSI users who would like a cheap set. It's a pretty small price for a huge benefit!
Home row Ctrl is center stem, as it is on HHKB. I hadn't seen a stepped Ctrl key before but I feel like it's too niche as both home row Ctrl and stepped caps are supported and I wouldn't imagine a lot of people mix the two.
Considering that this set is identical to originative, could it be considered a good idea to offer an addon including only the european keys?
No, addon kits failing to reach MOQ is the reason we almost never see some locales covered. Not to mention, this full kit will be far cheaper than Originative. If this set is very successful, it might be reasonable to run a similar set covering even more locales like French, Spanish, Portugese, Mongolian, Vatican, etc.