My candidate for the Benefactor

Never heard of this guy but still there are some questions that need to be asked. Why would this guy have to hide his identity and would he be connected enough to hear about the reapers? Also did he have the kind of funds required to keep the project moving forward? Not saying you are wrong but this seems so obscure but then again we are not given a lot to go on nor will we be getting an answer so why not.

fun fact, its still Cerberus

Actually, I'd place my vote for Aishya Ashland, Jonah's daughter who has been, according to Jonah himself, spending his fortune like water... and she's just flakey enough to fund a "hipster" project like The Initiative.

The Refund Guy Corporation is the benefactor. The refund guy seen in the trilogy? He is one of many. They go to different vendors claiming they bought a product that doesn't work and demand a refund. Have X number of folks doing that each day for many years, and they would have more than enough credits to fund the project. Obviously a few stayed in the Milky Way while others went to Andromeda. How did they know about the reapers? One of the refund guys is a former Cerberus worker who overheard TIM talking about the reapers. When he joined the Refund Guy Corporation, he mentioned it. It was later decided the Corporation would use the credits to help Garson build her ships.

Seems like Electronic Arts Entertainment?

But to answer the OP, might be someone from the Nexus completely unknown to us, or Tann because, you know, salarian?

The only reason I can think of for hiding the Benefactor's identity is so they can do a big reveal. When they're unveiled they want a 'Wow!' not a 'Who?' from the fans.
Unfortunately, as the pool of well known potential candidates is so small (Cerberus and Liara), they'll probably get a 'Meh.'. And everyone knows it's Cerberus anyway because Mac Walters.

It is suggested that the Benefactor knew SOMETHING bad was coming even before Shepard saw the beacon on Eden Prime.

Jack Harper, Desolas Arterius, Saren Arterius, Eva Coré... who did I miss and how many are actually alive (and not Reaper-crazed) by 2183?

They saw the Husks. Desolas was after a bloody artifact that was REAPER IN ORIGIN.

You're not convincing me otherwise, it's Harper.

Why though? They are a race that operates on logic, but this plan is illogical if they are the Benefactor. 

I don't know. I'm only half-serious, but I imagine I can spin a halfway decent theory about it.

They foresaw the coming destruction, I guess, and chose to act. It was their array, staring into dark space, that eventually was used to locate the Heleus Cluster. We're never told how geth knowledge or technical applications made their way into Ai hands. Quite the opposite, it's emphasized how mysterious it all is. They could easily come up with the money to back the Initiative; more easily than anyone else, realistically, seeing as they're the most sophisticated synthetic intelligence in the Milky Way proper.

As to why they'd have Garson killed, they'd do it for the same reasons anyone else would: to protect themselves or their secrets. Maybe they even chose Tann, for some reason. Everyone was shocked and puzzled when his name was spit out of the system as the new Director. Geth could easily tamper with such a process while leaving no one the wiser. I'm just throwing that out there, since Tann's ascension seemed orchestrated by someone.

The last thing this has going for it is that the Benefactor storyline could be wrapped up in Annihilation if it involves the geth. We'll see, I guess. If it ended up being true, I imagine thousands of people would emerge claiming that they saw it coming. As far as I can tell, though, I'm the only one promoting the (crazy) theory.

Right or wrong, it would be a heck of a lot more interesting than "Cerberus", "Shadow Broker", etc...

Nah. It's J.Jonah Jameson. He loves his conspiracy stuff.