Hi,
I should correct in one point:
The other advantage of using stockable variants is there is no need to upload images or descriptions to the children as they never display.
The same idea applies for "Generic Child Variant" as well. If you don't touch any thing on the child product, the parent product's information will be used for child as well. Therefore if you don't upload picture to child product, the parent's image will be used for child product(s).
About the problem you talked about. I see it (in our DEMO). it is very unusual situation. It is a problem and must be fixed immediately. I just don't know how possibly this problem hasn't been raised before.
A good news: in the upcoming version of CleanMart, the "VM - Custom, stockable variants" support will be added to CleanMart. Indeed there will be lots of new features as well. Such as; adding one more "product details page layout" and other tweaks and improvement in some sections. The next version of CleanMart will be huge! Hopefully it will be ready to be released this week.
But just for now. You can do what you have said: you can use some CSS rules to get
at least nicer add to cart button for "VM - Custom, stockable variants"
Regards.