There's no such thing as "Damage to Demi-Human is capped at 100%". It's just the calculation is a little complicated.
For every damage to demi-human card you put. The cards gives -5% instead of the original stat it gives.
Example:
On shoes: FBH Card Gives 50% to demi-human, if you put another FBH Card, the second card gives only 45%.
On weapons: StalkerSB gives 30% demi-human, if you put a goldering card it'll just give 20% since -5% is already applied because of the bow itself already gives more damage to demi-human. if you put another goldenring it'll only gives 15% so it's better to use Tgen and civil servants/skel workers.
And Yes, Goldenrings do stack with 2 FBH. proven and tested :)
Stacking calculation is only applied on each equipment not overall