MTB crankset for the same BSA 68mm like on the Birdy have a wider chainline due to the shape of their spider. There is no need of a longer axle.
If you begin to add spacers and use a crankset for a wider BSA bottom bracket box (like 73mm) you don't know what chainline you will actually get and if it will fit for the front derailleur.
About the Litepro chainrings, some people mounted them on the Brompton and had problems with the teeth shape.