"... how would we reach consensus..."

I don't think consensus is attainable. I believe that, for the foreseeable future, at least, conflict and tension between opposition parties is inevitable.

"Would your proposal then be an unwritten social contract?"

I propose no unwritten contracts, but only interpersonal agreements to which individuals set their hands, and can withdraw from at their sole option.

Unwritten social contracts are, I think, addressed by this proposal, as the principles of most folks recognize such, and their agreements would reflect their understandings.

You and I could agree on much. We would continue to disagree on other things, but the areas of agreement could be matters where we could work together, until our work ran up against one or the other's conflicting principles, at which point we could affably disagree, limiting our communal effort.

If those limitations proved to too limit the effection of our purposes, we could also choose to change our minds, and undertake a broader range of agreement.