I don't think it is a title per se but there have been at least 2 people who have gone by the name of Hoid. In the first sample chapter for Dragonsteel there are 2 people talking, one is named Hoid and I'm pretty sure he is dying (it's been a while since I read it), the other is named Mindus Midius. Hoid dies and I don't remember what happens next but I do recall it being said that the Hoid that we see popping up all over the place is actually the Mindus Midius dude. In WoK he even says that Hoid isn't actually his real name. I'm pretty sure that when they say hoid was around when Adonalsium shattered they mean he was involved in the circumstances of what caused the shattering as Dragonsteel is supposed to be the story about when Adonalsium shattered, and is also the story about the orgins of Hoid. It just wouldn't make sense to not have them be directly related to each other.
The 16 Shards are just that, a purpose. Well actually they are intents, but it's pretty much the same thing as what you were suggesting. That being said I don't see how there could be a 17th piece that was not a Shard, the 16 Shards don't really have any physical form after all, a new intent/purpose would just be a new Shard.
As far as we know, the Seventeenth Shard is just an organization that exists within the Cosmere. That's it, we don't know who's in it or what their goals/intentions are. But I don't think that they are trying to keep the Shard seperate. Rather I think that they want them to get back into one complete Something. I'm pretty sure Hoid is a part of the society and those seem to be his goals at least.
EDIT* Fixed spelling of name