I don’t think an ARC request can be denied on grounds that one signature is missing. The signature is more of a form of notice rather compliance with the CC&R or bylaws.
Submit the ARC form noting conforming building materials with the two signatures. You may be pleasantly surprised.
The HOA is most likely just responding to the complaint from your dick neighbor. He himself may also be an ongoing problem in the community.
Can’t see it being denied.
One thing though. You say it it build on a slope. Maybe the neighbor is fearing it may fall? I would include foundation details in your ARC addressing this as well.
