Could competitive flow reduce internal thoracic artery graft patency?

Yes: If the internal thoracic artery is used as a bypass conduit to a vessel that does not need a bypass, like one with less than significant blockage, or one that has adequate collateral flow to it, the bypass graft will gradually become atretic (small) and eventually shot down. In rare cases, it will close rather quickly in the immediate post-op period. This is not a common occurrence thankfully.

Absolutely: It can happen. If the vessel that is bypassed has significant flow, the mammary artery can fail.