God can do anything He wants. Omnipotence is, well, all powerful. The question is not what CAN God do, but what has He done and what does He want of us?
God cannot do anything He wants. Omnipotent means all powerful but that does not give Him the ability to deny Himself, lie, or change.
That argument is a false premise and anything built on it is not going to come to a correct conclusion.
Whatever God did to cause Mary to allegedly be born sinless to sinful parents, He could have done for Jesus to be born of a sinful mother.
If Jesus had to be born of a sinless mother to remain sinless when He is God Himself, then in order for Mary, a mere human to be born sinless, her parents would have had to be sinless as well, and back and back....