Go back in time and tell the contractor to fix his fuckup?

Otherwise, tape and mud and repaint.

Use the vacuum cleaner with the handheld brush attachment from both sides?

I would cut it off and install an Anti-Siphon Sillcock Frost Free Outdoor Faucet. It will save you any headache.

metal box = metal extender.

Just slid it under the vent half way, and just leave the pipe exposed (or hire someone to move it).

Op says, "I have no hardware to do that. Any other way?" Hire someone.