Falsifying it is as impossible as proving it, at this point, without a direct reveal from Brandon (which I don't see happening).
But the author could be any other Shardholder, really. Or anyone else with the ability to travel between worlds.
Oh, I'm firmly of the belief that the person writing the letter is Hoid. That interlude is the best evidence around, although there is plenty more. You're overstating your case, Munin.
The person writing the letter has the following attributes:
(1) Knew the Shardbearers personally before they bore the shards. This is supported by the references to Ati and Rayse.
(2) Has observed the events on multiple worlds. References to Sel and Ati make this clear.
(3) Has a quest of some sort, some goal he is trying to reach, which involves personal motivations against Rayse and Bavadin.
(4) Is being chased by the 17th shard, but has successfully laid down a false trail for them.
(5) Seriously needs help.
(6) Is directly involved in the events of the war that is presumably coming.
Hoid satisfies all of these conditions easily. This is more rigid than you might think; Sazed, for instance, is ruled out by condition (1). Most entities who would satisfy conditions 1-3 and 5-6 would have to be somebody new, somebody we have never yet seen. This isn't impossible, but we just don't know enough to propose anything. Condition (5) does suggest that the author is not uberpowerful, but given what Odium has done this probably doesn't mean much.
By contrast, condition 4 is ridiculously specific, and exactly what we see in the interlude under discussion, where the searchers look for Hoid by name. This point alone suggests Hoid as the author, barring really strong evidence otherwise.
Also, Hoid has been seen to directly interfere on Shardworlds, whereas we have no canonical evidence of other interference, so condition 6 suggests Hoid much more strongly than it does any other entity. Condition 3 would go a long way to explaining Hoid's behavior in other books.
In addition, just how many planethoppers are there floating around? All we've seen is Hoid and the 17th shard. The reference pool really is quite small here.
Maybe I'm missing something about Hoid, but how do we know that he fulfills conditions 1 , 3, 5, and 6? So far, he hasn't seemed like he was seeking help. As far as I'm aware, he hasn't expressed personal knowledge of any of the shardholders. He hasn't mentioned any motivations involving Rayse or Bavadin. And we don't know for sure if the people looking for him were from the 17th shard.
And it's unclear if he's even directly involved. At best, he's been an observer so far. A sarcastic observer, true, but that's basically it.
Hmmm. My support for #1 and #3 are from Brandon's statements about Hoid, not from the text of the books. Apparently, Brandon said Hoid was there when Adonalsium shattered, which makes him a very good fit for #1. He also said that Hoid had his own goals and motivations that he was pursuing in the background of the other shard-world stories. Basically, Hoid is not a passive character. He just never ran into the main characters before except in passing. That's why #3 is a good fit.
Note especially his discussion with Kaladin, where he said he came "there" (presumably, Roshar) in order to find an old friend, but now he spends all his time avoiding him. (This is compatible with him avoiding Odium, or even the person the letter was to.) Hoid is doing something. He's not just an observer. Or do you think he name-drops Adonalsium just for kicks? (Incidentally, that negates your argument about him not knowing anything about the shards. Anyone who knows that name is involved.) No, regardless of what else you think, Hoid is an active character.
#5 is compatible with Hoid. It doesn't rule him out. I was just trying to carefully distill the contents of the epigraphs.
#6 is also compatible with Hoid, especially the way he showed up in the right place and the right time to greet the last Herald as he returned. Again, it doesn't rule him out. His actions, when interpreted in the context of #6, make sense. Yes, he could have been there for some other reason (do you really believe that?) I mean, his whole conversation in the epilogue is headed towards the ending of the book---he was waiting for the Herald to show up, and seemed dismayed at the trouble he was bringing with him. No, Hoid is up to his ears in the War, even if you don't accept that he's the author of the epigraphs.
As for the people searching not being the 17th shard, give me a break. Get real. I've seen this kind of argument before on the internet, and I don't intend to get stuck in it because it doesn't go anywhere. Can't prove a negative, that kind of thing. You can't prove to me that I'm not a hallucinating turnip, either. Seriously pointless. I grant you the technicality: Brandon didn't come and say outright that the searchers were from the 17th shard; he just implied it heavily. So what? That doesn't change the fact that that's who they are and Hoid is who they're are chasing. As I said, #4 is the real kicker for showing who the sender is, and this is why.
This is a classic case of Occam's razor. Assuming that the author is Hoid doesn't answer all the mysteries, but all the pieces we do have, fit. Assuming it is someone else leaves pieces dangling all over the place, to absolutely no purpose, narrative or in-world logic. Rather than having two sets of planet-hoppers (Hoid, 17th shard), we would have three (Hoid, 17th shard, whoever the searchers in pure lake were). Rather than having one set of planet-hopping searchers lost, we would have two lost groups of searchers, in almost identical situations, just with different targets. (One would be hunting for Hoid, and the other for the mysterious author.) Hoid would be on Roshar trying to stop Odium (assuming Hoid is doing anything esle seems... unlikely), and so would this unknown author. We create double sets of extremely distinctive traits simply for the sake of not making assumptions... not bloody likely.
