Is everyone who attends the marriage contract considered a witness to it, or is that limited to those who sign the contract, given the presence of witnesses who do not pray?

The intent behind witness testimony in marriage (nikah) is for the two witnesses to hear the offer (ijab) and acceptance (qabul) between the two parties to the contract. If this is achieved, then the testimony is valid. It does not matter if one of the recorded witnesses is a fasiq (transgressor), because the testimony has already taken place in the presence of a gathering of people, and the contract is valid and cannot be annulled.