What is the real difference between Fr Chazal's position, which I think he refers to as sede-impede(?), and the Cassiciacuм thesis? Is there a practical one?

You will find it very ably described in his book, which contains two chapters refuting sedeprivationism, and reaffirming his classical R&R position.

No, he most certainly does NOT hold the "classical" R&R position.

Classical R&R holds that the commands/teachings of the V2 popes must be obeyed on a case by case basis, and rejected if they are bad.

Fr. Chazal's position is that the V2 popes are "impounded" or "quarantined" and lack any authority, and Catholics should completely ignore them ... on account of their manifest heresy, but thy remain the "visible" head of the Church until they are removed by Church authority.  So a visible head without authority = sedeprivationism in a nutshell.

These are two completely different things.

Fr. Chazal, despite his protests, unwittingly articulated a slight variant on sedeprivationism and the difference is very difficult to spot.

I wrote an extensive thread proving this by quoting him from an hour-long presentation on his position.  I will try to dig it up.

He independently (to his credit) came to the same conclusions as the sedeprivationists, for the same reasons, but then he got cold feet when this was pointed out ... because he appears not to have known about sedeprivationism.

Hallucinogenic wishful thinking.

As I mentioned at the time, I told Fr. Chazal what you were saying about him, and he responded that he refuted the very theory you persist in attributing to him.

Somehow, this has/had no effect on you.

So, with regard to my distinction between classical R&R and Fr. Chazal’s position, which do you dispute, my characterization of classical R&R or my characterization of Fr. Chazal’s position?

Answer this and perhaps there might be room for a rational discussion.

sede-impede

I've seen it called sedeimpoundism:

SEDEPLENISM -- See is occupied simpliciter.
SEDEVACANTISM -- See is vacant simpliciter.
SEDEPRIVATIONISM -- See is occupied secundum quid and See is vacant secundum quid.
SEDEIMPOUNDISM -- See is occupied secundum quid and See is vacant secundum quid.

Thoughts in red.

This is the false idea you and many sedes love to spread, why? Who knows? Whatever is Fr. Chazal’s position I cannot say, but the truth is that the R&R position is simply that "we are first of all under obedience to God, and only then under obedience to man". "He who heareth you heareth me" - God speaks through men, you reject this (Fr. Hesse) This means we Catholics remain under obedience to the pope Obedient in theory but not in practice?, but only so long as in obedience to the pope we do not offend God. And who's the final authority on what offends God - you or the pope?

Since the conciliar popes have commanded nothing and taught next to nothing we can obey without offending God, all we *can* do is remain alert and watch for (albeit do not expect) a command / teaching we *can* obey and embrace - without offending God. And this command will be identified by me / Bp. Williamson / Menzingen / the man I put in the place of God. This, in addition to praying daily for the pope, is what Catholics, faithful oh so very faithful subjects oh give me a break of the pope, do. This is the classic R&R position in a nutshell and I wish you would bookmark it.

If anything, it is the various different branches of sedeisms which are based on a type of case by case basis in regards to when the pope is pope and when he is not, you are confusing the various different sede ideas with classic R&R. 

Frankly, just an outrageous misrepresentation. You know very well that us Catholics only determine once whether a man is the pope and therefore to be obeyed and you have just admitted that you judge the Vicar of Christ's commands on a case by case basis.

Ladislaus tried to point out to you that Fr. Chazal says the pope is impounded, always, so all his commands are void, not just some of them that you feel like disobeying, so Fr. Chazal makes only one determination of credibility, just like us, as opposed to people like you who sift the Magisterium. 

A very simple distinction which is not entering your mind only because you obviously don't want it to.

At the end of the day, I don't care what position you take ... except if you claim that legitimate papal authority can corrupt the Magisterium and the Mass so badly that Catholics must refuse communion with and submission to him.  Those of you who believe this are not Catholic but are a slight variant on Old Catholics, with some hints of Eastern Orthodoxy and Protestantism.  Outside of these parameters, it matters little to me which of the "5 Opinions" you want to go with or how this situation happened, whether on account of Siri Theory (my personal belief), or due to manifest heresy, or defect of intention, or because Montini was blackmailed and not acting freely, or because Montini was drugged, kept in a dungeon, and replaced by a big-eared double.  These are all theories and details, and are peripheral to the main problem.  You can't throw the Holy Catholic Church under the bus, along with the repeated papal teaching that this cannot happen, along with the teaching of Vatican I that the Holy See cannot be stained or blemished by error, along with Our Lord's promises that the See of Peter will be the rock to which the Church will be unachored with unfailing faith, along with Trent's anathema against believing that the Church's Rites can be inducements to impiety, and the fact that no Catholic theologian has ever held that such a corruption of the Catholic Church is possible.  You even throw Archbishop Lefebvre's affirmation of the same principle under the bus ... while mendaciously claiming that you're the "true heirs" of Archbishop Lefebvre.