Well for one thing, it doesn’t matter one bit what Bishop Fellay has to say about the SSPX being in schism as he is a bishop of the schismatic SSPX. He has no standing within the Catholic Church to make any statements as to what the Church teaches or says.
Here is a link for you to some letters sent to the Vatican and their reply.
jloughnan.tripod.com/ratzprl.htm
In this letter there are a couple of points that I will post here, go tot the link to see it in whole.
As to what formal adherence means.
c. Thus far the Church has not officially declared what constitutes " formal adherence to the schism " inaugurated by the late Archbishop Lefebvre (cf. Ecclesia Dei 5, c), but the Code of Canon Law defines schism as " refusal of submission to the Roman Pontiff or of communion with the members of the Church subject to him " (canon 751). The above citation together with the other documentation which you have included in your dossier and your own exchange of correspondence with Father Violette clearly indicate the extent to which many in authority in the Society of St. Pius X corroborate that definition.
As for if they are in schism or not.
b. While the priests of the Society of St. Pius X are validly ordained, they are also suspended
a divinis, that is they are forbidden by the Church from celebrating the Mass and the sacraments because of their illicit (or illegal) ordination to the diaconate and the priesthood without proper incardination (cf. canon 265). In the strict sense there are no “lay members” of the Society of St. Pius X, only those who frequent their Masses and receive the sacraments from them.
While it is true that participation in the Mass at the chapels of the Society of St. Pius X does not of itself constitute “
formal adherence to the schism”, such adherence can come about over a period of time as one slowly imbibes a schismatic mentality which separates itself from the teaching of the Supreme Pontiff and the entire Catholic Church classically exemplified in A Rome and Econe Handbook which states in response to question 14 that
the SSPX defends the traditional catechisms and therefore the Old Mass,and so attacks the Novus Ordo, the Second Vatican Council and the New Catechism, all of which more or less undermine our unchangeable Catholic faith.
It is precisely because of this schismatic mentality that this Pontifical Commission has consistently discouraged the faithful from attending Masses celebrated under the aegis of the Society of St. Pius X.
And lastly, your suggestion and the Latin Mass Magazines interview with a bishop of the SSPX.
e. We reiterate what we stated above: “The Pope is the supreme legislator in the Church.” Communion with him is a fundamental, non-negotiable hallmark of Catholicism which is not determined by those who set themselves up to judge him, but by the Pope himself (cf. Second Vatican Council’s Dogmatic Constitution on the Church Lumen Gentium#22-25).