DY, you say “how can you assist someone do something assur yet not be oiver on lifnei iver?”
Not quite when Talmid B”H helps talmid B”Sh with tzaras bito he is helping him to something muttar! The way I see it even the acording to Talmid B”H, the talmid B”Sh is in fact allowed to marry tzaros bitto. If it was assur it WOULD be lifnei issur.
(As far as the paradox of not being lifnei iver for the machshi to help do somehing assur for the machshei to do himself, there are many such examples, nazir helping regular person drink wine, kohein helping yisroel be metameh. In ALL these cases there is no issur helping the nichshol do something muttar.)
I agree with your summary but with the caveat that I’m only talking about cases where the machshi holds the nichsol is allowed to do “it” (C”S/ carry with Eruv/ marry tzaros bitto) on his own, thus making it mutar for the nichshol, even in the machshi’s opinion.