As citizen of central European country with English as my second language (and I am far from good enough writer or speaker in English), I consider mistakes which are done by Zodiac as weird. At first I thought they may have been caused by something like him being dysgraphic and other mistakes seemed more like if he has foreign background…. but as much as I look at them, it seems they are not.

Lets just see spelling mistakes done in this letter and pick the letters up:
"ChristmasS" – S
"extreAm(E)ly" – A (missing E)
"dif(F)icult" – missing F
"ninEth" – E
"pos(S)ibly" – missing S
"victOm" – O instead of I
"drownDing" – D
"trig(G)er" – missing G
"controOl" – O

Is it possible that he made a mistakes throughout all his letters and that they might be one of "keys"? Note, that mistakes in "Belli" letter are contained mainly in middle part of the letter where he could easily wrote 9th and 10th…. but no, he wrote nineth instead of any other version of the same. Also – word control is written multiple time in thee same letter – only once with "mistake", other is written correctly. I also have to think about how he phrases some parts – why he wrote "wont" and then "can not" (which is not consistent and therefore interesting) instead of will not or cant…. Placement of mistakes in letter and which words contain these mistakes means something IMO. "Safe, sid go…" might have been message for someone.