I think what is going on here are two different issues. Normally the Catholic Church instructs us that we are not to participate in or attend the worship services of those Denominations, (including those who call themselves “Non-Denominational”) who have cut themselves off from the Church. However the Church does make an exception for special occasions, weddings, funerals, etc. In cases like this we are attending not to show our agreement with or communion with the false teachings of another Denomination, but to honor the bride and groom, or the life of someone who has died. We would not participate in the service in an active way, such as receiving communion, or reading from the pulpit etc. However sitting and standing when the congregation does so, and saying our own private prayers for the newly-weds when the minister is leading a prayer for them is permitted.
As for the Churches teaching on Marriage, when two baptized people marry, they are who preforms the sacrament. In the Catholic Church the Priest or Deacon represents the Church to wittness and bless the union, but it is the couple who preform the Sacrament, and Jesus who offers the Graces of any Sacrament. Unless the vows are changed to say something like “As long as we love each other.” or “until we find someone else” If both parties are baptized, and married in their Church, we have no reason to believe that they are not married.