I was speaking from the point of view of you responding to her jabs with your own thoughts and opinions on what happened etc...in hopes that some impact will be made on her (which could simply be the hope of her gaining clarity/understanding or for her to feel the hurt you feel from her decision to leave).
In my opinion the short response sent would make more impact than the long letter would have.
I dont know what she thought about it your reponse... but if she gathers anything negative from it (Just as she was "surprised" at your "lack" of concern for her car breaking down) hopefully she will start to see the negativity is coming from within and not from you Where as sending the long letter gives definite amunition/reason for her to continue to respond from a place of hurt as it drags up those emotions.
For me personally stepping in her shoes, (and totally assuming! ), the short response would sting a bit. Because I feel she is wanting that big "reaction" from you for whatever reason....and shes probably confused as to why she doesnt have the power over you emotions anymore...thus probably causing her to wonder what is really going on in your head and may in turn cause more self introspection for her. Which would be positives in my opinion.
me: 30 H:30 tgthr:7 m:4 no kids 5 counseling sessions initiated by H as a LR: Oct 2012 long distance marriage b/c of work since Nov 2012 official BD: July 2013 nothing filed 1/1/14 I dropped the rope