The current chain tensioner mounted on all Birdy 3 from R&M and Pacific Cycles works with a double chain-ring and front derailleur: it was factory mounted on my Titanium Birdy that has from factory a compact 52-36 front cankset with a front derailleur (FSA SLK-light crankset and Shimano 105 front derailleur).

For Birdy and Birdy spare parts from Germany, you can look on Ebay Kleinanzeigen Deutschland.

The Birdy is less popular in Germany than in the past because Riese & Müller doesn't promote it much and most current R&M resellers are specialized in high end eBikes not in folding bikes.

The rack you show has the drawback of increasing the folded size.

Why not the folding rear rack ?