If Mary had no 'original sin', then Christ had none and He could not be the perfect sacrifice for all of humanity (but only for Mary alone) because He could not have conquered 'original sin'.
"That was His Plan. Who are we to murmur against God and try to second-guess Him? That would be the ultimate in folly."
Doesn't that assume that you have deduced the Plan correctly in the first place? Is that not folly in itself?
As to folly, we trust what we read in the NT about promises to the Church. So it’s not so much Moi who got it right, but God speaking in his Church. FWIW.
As to Jesus and original sin. He doesn’t have to be infected, does He, to conquer it. The first Satanic invasion was successful. God comes, as Jesus, into occupied territory and successfully repels all the adversaries assaults.
Then, in dying, he enters the adversaries den and whups him there. Before Jesus only sinners were in Sheol. Hell and death had never seen anything like THAT before and were confounded and put to flight.