Thank you. I did try messing with levels - though I think I asked it to start at 6 or 7, and to stop at a smallish patch. The idea was "do not touch the big picture, fix small differences". For some reason this was taking ages to calculate and I ultimately stopped it and re-ran from level 0 which ran considerably faster.
As for 3dNWarpCat - if this is supposed to work as I tried, I think I will first do more checking. Maybe I made a dumb error somewhere.
To make sure that I understand:
If I want to make a T2 -to-T1 NL warp compatible (catenable) with a T1-to-template NL warp, resampling the T1 target of the T2-to-T1 to template resolution should suffice, correct? (supplemented with zero padding if necessary to keep everything fit in the image
