It's great that your SO is willing to suffer with you. My wife laughs at me when I even hint at her throwing a leg over a bike. I think she has seen me suffer too many times to get excited about the idea...

I have been in your situation. It can be tough. The good news is that the improvement will be rapid and this may sort itself out. Is a quick trainer ride one evening a week (or two...) something your SO might agree to?

My suggestions would be to make it clear to the group what the ride will be. If there is an appetite for more then an optional climb, as you suggested, might sort that out. This only works if you have the roads to make it work.

In terms of a prize, I ran a Tuesday night ITT thing here for a while. We kept track of times and gave two prizes: The first prize was for the best time in each cat (A and B groups). The second prize was for the most improvement. This meant that whomever had shown the greatest percent of time reduction, regardless of their group or placing, also won a prize. I felt that this rewarded effort and not just results. With a beginning group this might spread it around a bit more and be a little less A-Type about the whole thing.

Good luck.