My thoughts exactly dan.
Generally I think LE will have their internet forensics dept who will scour the net and get everything, but I wonder... I think these posts are pretty significant. It reinforces the idea of premeditation.
Indeed it does suggest premeditation. The flipside is that it could also be used as part of his defence - ie that he should be found "not guilty by reason of mental impairment" at the time - and that these posts showed he himself thought he was mentally ill.
As to whether such electronic material can be used in evidence, that will depend on 1) whether it is relevant to a fact in dispute; and 2) whether it is admissible. I would say that the large portion of this case will be proven by adducing electronic evidence.