"To stop" simply means to cease doing whatever you're doing ("I filled the jug until it was half-full, then I stopped"). "To stop oneself" has a connotation that you made a conscious decision to resist a temptation or instinct ("I wanted to throw your present out of the window, but I stopped myself."
Sometimes the final S in "tous" is actually pronounced - for example I have heard it in the sentence "ils sont tous la" (sorry - no accents on my keyboard). It's a choice, as all of the elisions are technically, except in the instances of the "h aspire" - you never elide the S in "les huitres." Generally speaking, it is considered more refined, or used to be anyway, to make the elisions. If you pronounce the S in "tous," even before a consonant, sometimes it makes it easier for the listener to understand that you are saying "tous" rather than "tout."
You have to pronounce "touSS" when it means "all of them/us" and masculine.
nous sommes / ils sont tous -S- ensemble, tous -S- unis, tous -S-à la gare, tous -S- là, tous -S- partis
elles sont toutes -Z- ensemble, toutes -Z- unies, toutes-Z- à la gare, toutes là, toutes parties.
But in which mode is it here in the sentence? If it's "objective" in the "l'un l'autre" case (we are stopping something else - the other one, not ourselves), then why does it have "être" as the auxiliary verb, which should go only with reflexive sentences? I'm gonna be downvoted into oblivion with my philosophizing, I know :D
Reflexive verbs are all constructed with auxiliary "être".
- serions arrêtés is past conditional (would have stopped)
- l'un l'autre is added for clarity: "each of us would have stopped the other" is the meaning (chacun de nous aurait arrêté l'autre = nous nous serions arrêtés l'un l'autre).
Like you stopped a couple of different things or people neither of whom are included in "we" ? (did you stop both rocks from falling? we would have stopped both-- if we had had our rock stopping tools on us.) "Nous les aurions arrêtés." or "Nous les aurions arrêtées" depending on the gender of the things or people you stopped. It is no longer the reflexive verb s'arrêter, but simply arrêter with a direct object pronoun.