Well, one thing Cuм Ex clearly does, it gives a layman authority to withdraw obedience, etc.:
"7. Finally, [by this Our Constitution, which is to remain valid in perpetuity, We] also [enact, determine, define and decree]:- that any and all persons who would have been subject to those thus promoted or elevated if they had not previously deviated from the Faith, become heretics, incurred schism or provoked or committed any or all of these, be they members of anysoever of the following categories:
(i) the clergy, secular and religious;
(ii) the laity;
(iii) the Cardinals, even those who shall have taken part in the election of this very Pontiff previously deviating from the Faith or heretical or schismatical, or shall otherwise have consented and vouchsafed obedience to him and shall have venerated him;
(iv) Castellans, Prefects, Captains and Officials, even of Our Beloved City and of the entire Ecclesiastical State, even if they shall be obliged and beholden to those thus promoted or elevated by homage, oath or security; shall be permitted at any time to withdraw with impunity from obedience and devotion to those thus promoted or elevated and to avoid them as warlocks, heathens, publicans, and heresiarchs (the same subject persons, nevertheless, remaining bound by the duty of fidelity and obedience to any future Bishops, Archbishops, Patriarchs, Primates, Cardinals and Roman Pontiff canonically entering)."
For my part, I wouldn't call withdrawing obedience, avoiding as a warlock and heresiarch, etc. as "doing nothing." That's lawful rebellion.
Right.
(musingly) How does one submit to the Roman Pontiff - without which it is impossible to be saved - when they are in rebellion towards the Roman Pontiff? 
Here again, you are ignoring the explicit instruction that DR posted from the pope promulgating in his official capacity as supreme head of the Church specifically to the laity, that we are not to obey those superiors who have deviated from the faith, not even the pope.
So first and foremost in the very beginning of cuм ex, the pope makes sure to explain to us that we must contradict a pope who deviates from the faith, then here above he tells us not to obey such a pope (or cardinal etc.)
This papal constitution therefore teaches three important things all Catholics are bound to believe, all of which you deny:
1) Cardinals and popes, contrary to your personal opinion-turned-de fide-doctrine, can indeed deviate from the faith.
2) They remain cardinals and popes who deviate from the faith.
3) We are not to obey them when they deviate from the faith.
^^This is what cuм ex teaches.
NOTE: #1 solves your conundrum:"The pope cannot fall into heresy, but if he does... but the pope cannot fall into heresy, but if he does... and on and on ad infinitum."
To answer your question:
"(musingly) How does one submit to the Roman Pontiff - without which it is impossible to be saved - when they are in rebellion towards the Roman Pontiff?"We should have to continue to obey him as the pope in all those religious matters which fall within the ambit of his authority, UNLESS he should command something which is sinful.
For Catholics, there is absolutely nothing whatsoever complicated about any of this.