hi Analect,
I believe you have mostly understood the
à la suite desquels: if [X] comes
à la suite de [Y] then X comes chronologically after Y, often due to the natural/planned flow of events and/or involving some degree of perceived cause-and-effect relationship.

So yes, the idea is that the owner will advertise that he has lost his horse, and that it will subsequently be found, recognized, and returned to him, presumably thanks to the awareness generated by his posters and other efforts. Whether the expression ought to be translated as "by which" in this particular sentence is another question entirely, depending on the nature of the translation job and the judgement of the translator as to how best to approach the passage as a whole.
Now consider the start of the sentence. We don't have the full context, but it rather seems that Fabrice may have had a role in the disappearance of the horse. Did he play a prank on the owner? Did he borrow the horse without permission? For whatever reason, he fully intends to reimburse the owner indirectly (
faire remettre) for the costs of recovering the animal. He also seems very confident that the horse will be returned and all will be well -- does he perhaps know where it is? In fact, I wonder if Fabrice intends to obscure the truth of whatever happened, arranging for the horse to be "found" by the local peasants and returned to its master, who, Fabrice anticipates, will have advertised for the missing animal. So depending on how many puppet strings Fabrice is pulling, the lost-horse posters might not really be the "means" of anything -- it all might just be an elaborate charade, narrated as Fabrice indends for the owner to think things came to pass! We would need the context of the whole story to know what's really going on here.
In the third-to-last sentence of the paragraph above, note how we can use "will have advertised" -- a future anterior -- in English to express planning about the relative timing of anticipated future events that have not yet come to pass.
This brings me to your second question. There is no "must have found" here, nor any supposition about past events, because none of this has taken place yet. Literally:
il se le sera fait rendre par les paysans qui l'auront trouvé
he will have caused it to be returned to him by the peasants who
will have found it
This tense communicates Fabrice's expectations about the anticipated future events: in his view, there is a high probability that the horse will be found and returned to its owner. Among its other uses, the futur antérieur is well-suited to this kind of anticipation of probable future events (although the relative timing dimension is less of a factor compared to English). See also the extensive discussion in
FR: futur antérieur récapitulatif et de probabilité
P.S. Apologies if my response in this thread back in 2012 confused anyone! At the time, I thought I was answering a question about transforming the last clause into "true" passive voice. In retrospect, it seems obvious that the poster was actually asking for help analyzing the causative faire structure. 