[kosta50]

a few words from our side are appropriate and often helpful since non-RCs seem more willing to accept (or at least listen to and think about) the Truth of what The Church has always believed from us rather than from them

Agree completely.

their dogma of the Immaculate Conception, something which, at least to me, calls into question the humanity of Christ

Precisely. The consequence of the Fall was death, not sin. Infants die because they bear the consequence of the Man's Fall from God. Our nature has the propensity to commit sin but one does not have to sin. The fact that Jesus the Man died on the cross means that he was born with that consequence. Yet, He was without sin.

If Mary was conceived immaculate and that is attributed to her sinless life, then her holiness is not an extraordinary effort; she is not ours to claim or to give to God (as we sing). Her holiness is magnified by her humanity and is not a passive and effortless consequence of divine will.

[Vicomte13]

Well put.
A Catholic need not disagree with any of what you said, and frankly, shouldn't, because it is exceptionally well said.