Master Spellthief applies to all your arcane casting classes. You don't necessarily add them all up in a pool (with Nar Demonbinder, anyway, which specifies levels in one other class. It's not specified if that's class levels or caster levels, but either way.... Knight of the Weave has less specific requirements, so it feeds back and gets crazy quickly). But for each arcane casting class you have, you add your Spellthief levels to it along with your levels from any other arcane classes.
With the build you give, you'd only have 7base+5wizard+4ASP=16CL for Nar Demonbinder - assuming class levels, not caster levels. But, for any arcane spell you cast from either class, you add the 16CL from Nar Demonbinder to the 22CL from Wizard, for a total effective CL of 38. If you assume caster levels instead of class levels, then you have 7base+22wizard+4ASP=33CL for Nar Demonbinder, for a total effective CL of 55, as you calculated.
The most confusing part is the way Nar Demonbinder words the stacking rules. I find putting it all into a spreadsheet helps.