My experience with the folding rear rack is the same: its excellent, it doesn't increase the folded size, it folds automatically and accept full size rear pannier without heel strike problems.

It can be loaded to 15kg without any issue on the bike ride.

Indeed, the bottom hook of the rear pannier like there are on Ortlieb and Vaude rear pannier hook into the rear mudguard mounting rod.

I use Vaude rear pannier and I have mounted two thin easy wheels for Brompton that still allow to mount the two pannier (the easy wheels foreseen by Riese & Müller are much too thick to be compatible with rear pannier) with those wheels, the folded Birdy roll on these wheels when pulled with its front part lift from the ground.

H&H has a lightweight titanium folding rear rack that mounts like the original one from Riese & Müller, its a very bad design, the titanium bolt delivered with this rack (with M3 very short thread !) aren't reliable and the plastic bushing of the rack wear pretty fast. Funny enough, H&H gives a max load of 20kg for this rack, more than what the Birdy bike manufacturer specifies for the rear rack on the bike he designed!