How sad that we are having a convo on saving the TourX. It is likely moot as it appears GM is killing off most if not all of Buick cars...but hey! It's fun to concept what could work, right?
I found your post thoughtful. The "Why" is nicely said. The "How" ...I wonder if rebadging across Chevy in this case is perhaps something to consider down the road, but for branding and differentiation purposes now, I'd maybe just leave it at Buick. But to your point [at least I think your point included this] ...maybe Chevy's marketing dollars could jumpstart both car and category. It would be great see Ford, Mazda, Hyundai, etc. bringing their Euro wagons stateside because GM opened up the market again with TourX, Nomad, etc.
I'm fine with Chevy getting the lower-spec model for $30k. Drop it from Buick. Keep the mid-level at Buick, but Essence would have packages 1,2 plus S&S all standard at more of a value price, say $36-37k. Make the roof the only option as some of us don't like less headspace / just don't need a roof. Keep Buick in the whitespace of very attainable near-lux while severely undercutting pricier near-lux peers like Audi/Volvo.
YES. Would be cool to have a GS / GSi  (hotrod) option with a V6, the GS seats and adaptive suspension with magnetorheological standard. Along with other cool stuff the Opels get that we don't. Like the Avenir thought too for super-plush types. I'm great with Essence, though.
Regarding Buick's ad agency...and I'd caution anyone this...don't be too quick to blame or namecall them. As a nearly 30-year veteran of the biz, I can tell you that great ad campaign ideas come from very hard-working ad agency teams 99.9% of the time. There is that rare instance when a client will have (or more likely, contribute to) a viable, cool campaign concept idea that actually has legs. Most of the time, though, great work happens via sincere partnership with a client wise and courageous enough to do their job in guiding and greenlighting good, if not great work. 
Most agency people just want to do awesome work that as one of my colleagues put it, is..."Work you want to put up on the fridge." We're creative people who work commercially because not everyone is a fine artist or bestselling author, and conversely, some of us are just damn good at creating and selling work that a) gets past client filters and the crippling maze of internal politics in both agency and client, and b) solves a problem for the client and 
in some cases becomes part of the fabric of a generation.
Last year I worked on a major domestic airline for a good part of the year as a freelancer — which I've been doing for 26 years this month. IMO, the client did not seem interested in even viewing much of the creative we presented which was very good work btw. Hard to fathom, but I am sadly (for all concerned, especially their brand) not kidding. Sometimes that fear of God needs to be shared on the client side as well. 
😉