You are trying to tie the Sanskrit meaning of the word Vishu to the agricultural festival that is celebrated on Medam 1. Sanskrit entered the Malayalam lexicon only relatively recently (i.e., in relation to Malayalam's and Kerala's long history) so your question can perhaps be answered as a cultural one. The easiest way to bring the native agricultural festival (which as you correctly point out should have solar ties) under a brahmanical/Sanskrit umbrella was to name it in Sanskrit according to its the closest solar event, the spring equinox. There may have been no need of absolute synchronicity for this naming. It would be interesting to know what the festival was called before it came to be called Vishu. I wish I knew the answer to that.