Astrid addresses that point, she says people dont know that the listener is dead, and that they eventually hear about who performed the ritual from the streets. They do have spies.

That would explain why it took them so long to get to Aventus Aretino.

It is not hard at all:The nordic tombs are open you can go in grab what you want and go out the corpses aren't even buried....Are you saying that someone who wants to contact a crazy murder cult is not able to do that?And if it is a noble he can hire someone to get the materials or buy them.

Note also that while the dark brotherhood brand is strong, it is not the sole presence in the market for assassination, e.g. the Morag Tong, some thug with a half-brick in a sock, the thugs that get contracted out to deal with you (the PC in Skyrim) whenever you wrong somebody, etc. The few people in Tamriel that go out of their way to have an enemy assassinated might consider some option other than the Dark Brotherhood.

The idea in Daggerfall was that they killed anyone else who entered the market, as well as the upstarts' clients. When the Dark Brotherhood is strong, using them is safer.

I might be remembering wrong, but I recall there also being a line indicating that they were aware of his contract for a while but chose to ignore it because they thought it wasn't worth it because he was only a kid.

Since there's instantaneous magical transportation, it stands to reason that there's also instantaneous magical communication, which would allow a single Listener to transmit any orders to the appropriate Speaker. Even if it's not instantaneous, assumedly the Brotherhood doesn't want to appear desperate by showing up as soon as the Sacrament is performed. I'd imagine the Speaker would take offense to an impatient client. If there are only four Speakers (and two of them are in Cyrodiil, circa Oblivion, with the Listener), then I don't think the guild's numbers are particularly numerous, and neither are the number of contracts they receive. But they are a "luxury" choice for assassinations: when you want a guaranteed result, with your specific instructions adhered to, you contact the Dark Brotherhood. Remember that the Falkreath Sanctuary is only having trouble finding contracts, not fulfilling them.

The Night Mother personally chooses her Listener, so presumably she can "see" who'd be the best choice: someone who will relay her orders appropriately and in a timely manner. Remember that the Night Mother doesn't necessarily tell the Listener every attempt to contact her: just the ones she thinks are worth the Brotherhood's time, I suppose.

It's not necessary that each member be the epitome of professionalism -- only the Speakers need to be, since they're the ones who actually meet with the client. Aside from that, the Sanctuary's "administrator" chooses the best fit for the job. Contracts aren't always secretive -- the client gets to specify the parameters, which can include public killings or particular methods (we have in-game examples of an assassination meant to be loud -- Vici's killing during her wedding reception -- and one intended to be as quiet as possible -- Baenlin's death, meant to look like a plausible accident). All of the Speakers and Sanctuary contract-administrators that have appeared in the games have been quite professional, if sociopathic -- dealing in death will do that to a person, I suppose.

I wouldn't see appearing early as being desperate. I'd see it as being quick, efficient, and more to the point, terrifying. You start calling for an assassin and suddenly they're right behind you? That's the kind of organization that can get things done.

Which I thought was weird. Wasn't it made clear in Oblivion that when the Sacrament was conducted, it wasn't to be ignored? This could've been foreshadowing of how Astrid thought herself above the Tenents.

Maybe she was busy with the sending that conga line of assassins after the Last Dragonborn. No wonder the organization is in such decline.

Also, maybe she gave Maven Blackbriar priority.