I take it you mean Paul means an absolute all. Because if you translate like that it would also include Jesus Christ, He being a true man as well as true God, good angels who never sinned, miscarried children, aborted babies, newborns, and severely retarded or brain-damaged persons, who do not have sufficient knowledge to commit actual sin.
If the translation is not absolute then your argument collapses. Because if all doesn't mean absolutely all, without a single exception, then Mary could be an exception (as indeed she is).
No peg because Scripture interprets scripture
There is no scripture indicating Mary was sinless.. Hbr 4:15 For we have not an high priest which cannot be touched with the feeling of our infirmities; but was in all points tempted like as [we are, yet] without sin.
Hbr 9:28 So Christ was once offered to bear the sins of many; and unto them that look for him shall he appear the second time without sin unto salvation.