Utpaladeva argues that if consciousness did not have vimarsha,then it would be inert and dead.How is this the case? and during pralaya is vimarsha active?
Yeah this is true just like there is no significance of Sun without its rays. Same is the case with Shiva, the highest consciousness. If Shiva is consciousness then his svabhava is vimarsha.
As Acharya Abhinavagupta puts it,
"If the Highest Reality did not manifest in infinite variety, but reminded cooped up within its solid, singleness, it would neither be the Highest Power nor Consciousness, but something like a jar"
So in order to manifest he has to have that svabhava(or Vimarsha). Mere Prakasa(Light) cannot be the nature of Reality. Even diamond is prakasa, but the diamond does not know itself as prakasa. Vimarsa is that aspect of prakasa by which it knows itself.