1-- There is NO SUCH THING AS A SEDEVACANTIST
2-- I agree that the NO church is not the Roman Catholic Church but it claims to be. A man claiming to be Pope who is not is an anti-pope.
2-- It is not up to Myrna to give us her 'point of view' re: the Fr GWS popes as anti-popes. As of yet No Church Authority has been cited telling us this.
3-- It is correct that the FR GWS popes were orthodox in belief. This is why( or one reason why) they were(are) NOT ANTI-POPES.
I pretty much agree with you, although while I don't think there were any anti-popes during GWS, we can't exactly go by whether or not the Church has declared them anti-popes. The Church, as far as I know, didn't even officially claim Pope Formosus an anti-pope when really he should have been declared one.
Actually we can( most of the time) go by whether or not the Church has declared an anti-pope. Possibly I am mistaken but we are bound to recognise a formally declared anti-pope just as we are bound to recognise a formally declared saint.
Examples of formally declared anti-popes can be found on pg 405 of Prof Chadwick's History of Popes.
" ... Pius X had to GET RID OF( Chadwick is a Prot who does not use the word anti-pope) four popes from the past-- ....Boniface 6....., Boniface 7, John 16.... and Benedict 10..."
SS is correct however that there are a few cases where it is still debatable if a Pope is actually an anti-pope. These are almost all found in the house of Theophylact. This is because historical accounts of events are somewhat vague.
I do not recall the circuмstances re: Formosus offhand but he may be one of those.