I personally believe the etiquette with alliances is that, the player that wishes to dissolve the alliance gives his partner/s at least one turns notice (i.e. he notifies them at the end of his turn that by the start of his next turn he will be (potentially) attacking them. This gives the former alliance partner/s a turn to adjust their defences this avoids a "stab in the back". Perhaps longer notification is more chivalric for chained or adjacent fortification – but it is not essential, you should be aware at all points of the game that alliances are transitory and can switch many times as the game ebbs and flows – therefore fronts cannot be left completely undefended (this mearly tempts an ally into turning against you [unless this is your plan
]). I believe the notified player should not attack the notifier in this period - if there is any contention then the non aggression could be extended to the end of the notifiers next turn - though I believe this is unnecessary.
The situation you have specifically raised though is when all other opponents have been eliminated. In this case I believe the former etiquette is obsolete. By eliminating all other players, your former partner has become defacto your enemy and it is ridicules to be held in a state of non aggression with the remaining player. There is no need for notification of dissolving alliances and all turn based extinction clauses become void also.
I would further propose that in some cases it would be justified to attack through your ally’s territory to finish off another player’s last territories. If it was certain that the remaining players would only be yourself and your (former) ally. I can see this being contentious, but if this is not the case then there can be ridicules situations where an ally will refuse access through their territory to kill an enemy as they insist on a turns grace to prepare for the final attack phase. That in reality means they have decided to ally (temporarily) with the former enemy against the ally - making the alliance null and void.
Personally I would never break an alliance, but it should never be taken that an alliance should be stuck to in all situations - there is a time and place for all attacks and ultimately any action which wins the game is justified. Everything can be forgiven with victory. If someone breaks an alliance with me I don't whine - I just remember in future that they can't be trusted
