It's 8kg per piece.

Unless your bags look huge you won't have anyone challenge you on a C ticket at FRA. The C bins on the 320s are generally pretty empty - you won't have a problem there.

I had to store my roller in a main deck bin the last time I flew upstairs. I haven't flown upstairs on the 744 since they removed the beds, so I will leave someone else to comment on how much space is available there.

It's hit or miss when checking in at out stations. Again, if your bags look oversized, you're more likely to get challenged. And sometimes you'll just plain get unlucky and you'll encounter an agent who simply insists on weighing the bags. The problem at DEN (if there is one) will be at the check in counter, not at the gate, so you'd end up checking the bag there.