Huh? Why do you say Mary can't be "truly of mankind" if she is sinless, but (apparently) assume Christ can be "true man" though He is sinless? You do realize that the Second Person of the Trinity is eternal, not bound by time, divine before all creation? Your post sounds as if you believe that Christ somehow became divine when He became flesh. You're free to believe that, of course, but it's not standard Christian doctrine.
And if she were sinless by special dispensation (read: divine favoritism), then (A) she did not need a savior (see Luke 1:47, “And my spirit hath rejoiced in God my Saviour”), and (B) Christ is not the only way. Precedent is set: why then condemn all for original sin if it may so easily be wiped away without sacrifice?
You've got to distinguish between time and eternity. Of course, Christ's was Mary's savior -- the special dispensation consists of His merits being applied to her at her conception.
In eternity, there is no distinction of past, present, future. Only time (which is itself a creature -- so God is not subject to it) is so limited.
Fine - why don’t we all get that “special dispensation”?
Nothing says she was sinless. “Full of grace” does not equal “sinless”.