Here the sequence:

1. The man commits the public sin of manifest formal heresy.
2. The man loses membership in the Church ipso facto.
3. The man loses office (and ordinary jurisdiction) ipso facto.
4. The man is excommunicated latae sententiae.
5. The man is declared by the Church to have incurred the penalty of excommunication latae sententiae.
6. The loss of office, that already happened ipso facto in No. 3 above, is enforced by the Church.
7. A new man is elected/selected to office.

Nos. 2 and 3 are a direct and immediate consequence of No. 1, and concern the Divine Law.

If we were to grant that Jorge Bergoglio was validly elected in 2013, and committed the public sin of manifest formal heresy some time during his pontificate, we would be awaiting the execution of No. 5 at this time.  However, the office is already lost with no claim to it on his part. 

I hold with the sedeprivationists (and with John of St. Thomas, Cajetan, and Fr. Chazal) that there must be some role for the Church to play, since it doesn't suffice for Fr. Cekada's "Aunt Helen" to wake up one morning and declare a pope to be a non-pope.  

Abusus non tollit usum.

No, but argumentum ad absurdum applies here.  There is no principled/theological backstop to prevent chaos in the Church, as explained by John of St. Thomas.

You claim that I'm "abusus"-ing it by holding that Ratzinger, Wojtyla, Montini, et al. are also non-popes by the same principles.

That Catholics are in chaos is the fault of the Church leaders and not the fault of those making the correct observations.

I did not say the latter and it doesn't follow.  Each individual must be looked at uniquely.