Slitting the throat in the bedroom, no idea. But evidence was presented that Brendan Dassey had bleach-stained jeans, and I think part of his confession was that he cleaned up the garage with bleach.
There were lots of different articles discussing evidence MaM left out of the documentary after it aired. Here's a few I just found:
http://www.independent.co.uk/arts-e...ery-the-netflic-show-missed-out-a6807961.html
http://www.independent.co.uk/arts-e...teven-avery-teresa-halbach-case-a6845066.html
Also, it should be noted that prosecutors don't need to prove a crime with forensic evidence every step of the way. That's what many juries want today in this "CSI" age we live in, but it isn't necessary. Certainly the defense can poke holes in the prosecutor's theory if he argues the victim's throat was slashed, but can't supply any evidence supporting that. Direct evidence isn't necessary to sustain a conviction, though, circumstantial evidence is sufficient in every jurisdiction that I'm aware of.