Regarding the pedo letter, apart from the content, which is plainly straight from Jodi's malicious imagination, the forgery itself is also so obvious.
Just a cursory examination of the handwriting reveals so many "tells."
In Jodi's handwriting, the descenders on letters extend far below the baseline, almost as a flourish. In the forgery, all the "Y"s follow that tendency, with the descender of the "Y"s falling well below the baseline. In Travis' known handwriting samples, although he uses an odd mix of upper & lower case, he consistently keeps almost all letters on the baseline.
Also notable is that with the letter "B," the middle stroke between the two curvilinear parts meet at the vertical stroke in Travis' writings. Jodi, however, has a lazier style, where her "B"s look more like a "D" with a bit of an indent in the middle. In the letter, the "B"s look more like Jodi's handwriting.
I don't know if any of this makes sense without being able to show the visuals, but I hope you can get the idea.
At the end of the day, this document is such an obvious forgery I can't believe that Nurmi said something to the effect that "experts" couldn't rule it out as being not authentic (or some such nonsense).