OK, jump on Jstor and google your topic, there are some good papers on it.
Yes, if you have access to an academic library, that ought to be your first port of call. The contributors on this forum are largely laypeople - such as myself - and so a niche subject such as late medieval ecclesiology is unlikely to garner much (name removed by moderator)ut.
That being said, I’ll try and answer your questions:
defining conciliarism as a heresy
How explicit of a definition are you seeking? Ecumenical councils (especially earlier in time) typically do not follow an explicit formula of “we define _____ as a heresy”.
Generally the most explicit identification of heresy occurs when specific practices and/or beliefs are noted with a corresponding canonical penalty (excommunication, anathema, etc.).
If you look at the section prior to the excerpt I provided, there is given (probably) the most transparent condemnation of conciliarism:
cum etiam solum Romanum pontificem prio tempore existentem, tamquam auctoritatem super omnia concilia habentem, tam conciliorum indicendorum, transferendorum, ac dissolvendorum plenum ius et potestatem habere, nedum ex sacrae scripturae testimonio, dictis sanctorum patrum, ac aliorum Romanorum pontificum etiam praedecessorum nostrorum, sacrorumque canonum decretis, sed propria etiam eorumdem conciliorum consessione maniseste constet.
[It is clearly undisputed that only the current bishop of Rome, has the full right and power to convoke, transfer and dissolve councils, as he holds authority over all councils. This is established in the testimony of the sacred scriptures, the statements of the holy fathers and the other bishops of Rome who are our predecessors, and the decrees of the sacred canons, and indeed also in the declarations of the same councils.]*
*Note, I cited the wrong volume of
Sacrosancta Concilia, it should be
19 (not 20).
The noun
consuetudo is often translated very literally as “custom” or “tradition”, but in Latin discourse it typically had a strong judicial tone to it, suggesting something as “customary by right/law (ius)”.