1) I would advise against using the term "patchwork", some might misinterpret your posts an attack on the Constitution, since it is used by Anti-Americans as a pejorative.
2) I think Madison explains why this idea is will not work, and is not in keeping with the Founder's vision of Federalism and Enumerated Powers per Federalist 45 :
" ... The powers delegated by the proposed constitution to the federal government, are few and defined. Those which are to remain in the state governments are numerous and indefinite. The former will be exercised principally on external objects, as war, peace, negociation, and foreign commerce; with which last the power of taxation will for the most part be connected. The powers reserved to the several states will extend to all the objects, which, in the ordinary course of affairs, concern the lives, liberties and properties of the people; and the internal order, improvement and prosperity of the state."
Abortion clearly is not within the narrow jurisdiction of the Federal Government.
I agree that abortion, per se, is not in the federal purview, but settling full faith and credit issues between the states is an enumerated power of Congress.
What happens when the states conflict with each other over their abortion laws, and an interstate issue arises? What happens when, for example, Texas tries to prevent a woman from crossing state lines to get an abortion? Will Texas be violating a woman's right to freedom of assembly? If a woman has an abortion in New Mexico and returns to her home in Texas, will Texas try to prosecute her? Does she have a "full faith and credit" defense arising from the laws in New Mexico?
What happens when Thomas' veiled threat to go after same-sex marriage happens? Will some states refuse to accept marriages allowed in other states? That's ripe for a "full faith and credit" crisis right there.
-PJ