My comments first: When a marriage that has failed and ended in civil divorce is placed on trial in an ecclesiastical court of the Catholic Church, the Catholic Church must make the determination if the presumption of the sacramentality of the marriage was present or not. In the authority given to the Catholic Church by our Lord to "loose and bind" the Catholic Church in justice may declare a marriage as a sacrament invalid from its beginning after the proper canonical trial that proves such a thing.
Grounds for an annulment which must meet the "proof" test of witnesses to include the former spouses would be 1) lack of due discretion at the time of the wedding; 2) lack of belief or understanding of the Catholic meaning of the Sacrament of Matrimony; 3) reserving the right to be unfaithful to the vows at the time the vows are made; 4) refusal to have children; 5) mental illness at the time of the wedding; 6) undue pressure to get married especially from parents or a desire to escape parents through marriage. There are many other grounds that must be proven too!
Sometimes there are no witnesses apart from the former spouses themselves. In these situations, the spouse's testimony should be sufficient!
Protestants are dispensed from the Catholic form of marriage. Their first marriage, even if common law is viewed as sacramental. I believe this is a grand mistake. The Catholic Church could declare these marriages simply unions, valid but not sacramental (meaning those that are done outside of any Christian denominational Church). A lack of proper religious form could be cited as the grounds to free a Protestant to have their second marriage convalidated.
A Catholic who enters marriage outside of the Church, even if free to marry, and does so without a Church dispensation is not considered to be in a sacramental marriage. No annulment is needed for this type of marriage only proof that the person was Catholic and decided not to be married in the Catholic Church. Something of this sort could be declared for Protestant marriages too!
As well when Protestant marriages end in divorce and there is a remarriage, and then one or the other wants to become Catholic, the annulment procedure for them should focus in on the Protestant's understanding of marriage as their particular denomination teaches. If it lacks what the Church teaches, especially as it concerns Holy Matrimony as a sacrament, then the annulment should be granted in an expeditious way without involving too many other Protestant witnesses who might be hostile to the process.
Pope Establishes Commission to make Marriage Annulment Procedures Simpler
On August 27, 2014, the Holy Father decided to proceed to the Establishment of a special study Commission for the reform of the canonical matrimonial process. Regarding this decision, the following is made public.
This Commission will be presided by H.E. Abp. Pio Vito Pinto, Dean of the Tribunal of the Roman Rota, and will be composed of the following members: H. Em. Card. Francesco Coccopalmerio, President of the Pontifical Council for Legislative Texts; H.E. Abp. Luis Francisco Ladaria Ferrer, S.I., Secretary of the Congregation for the Doctrine of the Faith; H.E. Abp. Dimitrios Salachas, Apostolic Exarch for the Greek-Catholics of Byzantine Rite; Monsignors Maurice Monier, Leo Xavier Michael Arokiaraj and Alejandro W. Bunge, Auditor Prelates of the Tribunal of the Roman Rota; Fr. Nikolaus Schöch, O.F.M., Substitute Promotor of Justice of the Supreme Tribunal of the Apostolic Signatura; Fr. Konštanc Miroslav Adam, O.P., Rector of the Pontificia Università San Tommaso d’Aquino (Angelicum); Fr. Jorge Horta Espinoza, O.F.M.,Dean of the Faculty of Canon Law of the Pontificia Università Antoniamum; and Prof. Paolo Moneta, formerly professor of Canon Law at the Università di Pisa.
The work of the Commission will start as soon as possible and will have as its goal to prepare a proposal of reform of the matrimonial process, with the objective of simplifying its procedure, rendering it more slender, and safeguarding the principle of the indissolubility of matrimony. [Original source in Italian]