Further support for sede-doubtism (courtesy of the CMRI):
Instititiones Theologiae Fundamentalis [1929] Rev. A. Dorsch:
“The Church therefore is a society that is essentially monarchical. But this does not prevent the Church, for a short time after the death of a pope, or even for many years, from remaining deprived of her head [vel etiam per plures annos capite suo destituta manet].”
The Relations of the Church to Society, [1882] Fr. Edward J. O’Reilly, S.J.
“In the first place, there was all throughout from the death of Gregory XI in 1378, a Pope — with the exception, of course, of the intervals between deaths and elections to fill up the vacancies thereby created. There was, I say, at every given time a Pope, really invested with the dignity of Vicar of Christ and Head of the Church, whatever opinions might exist among many as to his genuineness; not that an Interregnum covering the whole period would have been impossible or inconsistent with the promises of Christ, for this is by no means manifest, but that, as a matter of fact, there was not such an interregnum.”
The Catholic’s Ready Answer, [1915] Rev. M. P. Hill, S.J.
“If during the entire schism (nearly 40 years) there had been no Pope at all — that would not prove that the office and authority of Peter was not transmitted to the next Pope duly elected.”
The Defense of the Catholic Church, [1927] Fr. Francis X. Doyle, S.J.
“The Church is a visible society with a visible Ruler. If there can be any doubt about who that visible Ruler is, he is not visible, and hence, where there is any doubt about whether a person has been legitimately elected Pope, that doubt must be removed before he can become the visible head of Christ’s Chuich. Blessed Bellarmine, S.J., says: ‘A doubtful Pope must be considered as not Pope’; and Suarez, S.J., says: ‘At the time of the Council of Constance there were three men claiming to be Pope.... Hence, it could have been that not one of them was the true Pope, and in that case, there was no Pope at all....”