Thanks for this.
The dual drive does not affect the fold much. I can wheel either of my bikes and the 3X7 sach conversion is a doodle. You can run a 3X9 Sram with a toggle chain, you don not need the click box.

I do not understand why you need to change your crank. A 53 chain wheel can be bought for next to nothing used. You just need a 130mm BCD pattern. There will not be much difference between 53 and 56 tooth. 56 chain wheels are also obtainable at a cost from other placers if you search,let me know if you find a cheap source. Also can you not get a smaller rear sprocekt for the Alfine.

For me all the bike friday range does not fold as well or as quickly as the Mezzo,except the Tikit. But I find the Tikit too bulky when folded and its costs more. I bought the Mezzo wanted to do the bull bar dualdrive convertion and it has worked out well. The bike I have is a perfect compromise for me that ticks all the boxes.

Mezzo support is not great in UK either, but the spare parts arrive very quickly when ordered and are about the same cost as Brompton spares. I asked a question about brake calliper reach required by email and did not get an answer.

Anyone know what reach I require for the rear wheel callipers?