Well well well, looks like it's quite complex just to have a 'better' shelling command :).
I did some .igs export test in Viacad, works fine so far (but it was simple objects). However it's true the shelling wasn't working
so good when objects are not created in viacad itself ... I suppose the construction history might help to resolve complex overlapping surface.

Well I thing I'll give up for the moment, as it requires to much technical knowledge and I don't have the time (I'm a bit lazy when it's about
reading documents).

Looks like the first best friend for Moi is still Rhino (maybe not for the shelling), I hope I will find some job soon where I can use all I've learned
so far in Moi, and pick a Rhino license (a bit expensive for an hobby style of work)