The add mode currently does a straight add of all the closures. This means the transparency closures will also be added together which is probably not desired since this could result in opacity going to 0 and the curves being fully transparent. While that will clearly make the render look wrong, I'm not sure why that should freeze arnold. If you disable the min_pixel_width (set it to 0) and make sure the opacity params are kept at fully opaque, does that fix your issues?
As for this issue with add and transparencies, by happy coincedence we came up with a possible fix for that just yesterday, so assuming no unexpected surprises during our testing, we might be able to release that in an upcoming version of arnold.
