As intense as these accusations are, it's hardly fair to assume that everything said in that letter can be taken at face value.
I mean, just look at who wrote this: this is someone that (a) has a borderline obsession with Monty Oum, going so far as to admit that he was in this to work with Monty, not Rooster Teeth, and (b) he was let go from his job some time before releasing this letter. I don't know Shane from Adam, but these things lead me to believe that he has an ax to grind with Rooster Teeth.
On top of that, Shane's remarks in the letter are just flat-out unfair. He acts like he's the only one that loved Monty when we all know that everyone at Rooster Teeth loved the guy. Furthermore, he gripes about how Rooster Teeth aren't running RWBY the way Monty would've wanted, but what can you do when someone dies and isn't there to guide the team that works on the show?
Shane completely disregards guys like Kerry and Miles – people that have played a major role in working on the series for quite some time. Given that Monty is no longer there to work with all of them to hammer this stuff out, what other choice do they have? Leave it all in Shane's hands 'cause he was apparently Monty's closest protoge and no one loved him quite like him?
As for this business with Sheena, I am indifferent on it. There's certainly a chance that RT and her haven't been on good terms do to how Burnie conducts business, but I don't see why they absolutely had to bring her in as some kind of guiding force behing RWBY. Why should she be trusted with being the new director or influence behind the show? Because she was Monty's spouse? Because Monty wrote a role for her in the show?
Do I think Rooster Teeth handled this perfectly? No, but I think they made due with what they had and did what they could to make sure that the show/IP could move on – just as Monty would've wanted. Heck, a lot of fans think Volume 3 was a big improvement over Volumes 1 and 2, so maybe they're on to something.
Again, I think Shane has an ax to grind and a bone to pick with RT and he's using Monty's coffin as his personal soapbox to do all of this. The only people who are really backing him on this are Sheena, who might have personal beef with the company over some dispute that we're in the dark about, and Kathleen Zuelch, who left the company on questionable terms over 'lack of support' from RT and butting hands with Barbara.
Unless we hear from the RT folks (which I doubt we will 'cause there's likely a NDA in effect, from Shane being let go), we'll never know what the whole story is.