Is the signature of the two just witnesses on the marriage contract sufficient for its validity, even if one of them did not attend the ceremony of concluding the contract, and is the contract valid thereby?

One must turn away from misgivings and pay them no heed, for the fundamental principle regarding a Muslim's affairs is soundness, and certainty is not dispelled by doubt. Seeking Allah's help and frequently supplicating Him are among the greatest aids in getting rid of them.