Are sublist in case Record (message sublist mainly) not accessible in suitescript? Even though their are mails, the count is coming as 0.

I haven't tried it, but the alternative would be to use 'N/search' to find mails attached to the case.

Trying to implement some requirement which requires userevent script and access to case record emails at the time of case creation. Search will not work.

how are you creating a case with rows in its message sublist

Using the search module should work fine in a userevent script to access the emails. It might also be a sublist subrecord which is accessed differently.

Some cases are replying to our no reply email. Sent through our inhouse app. And when they do the case gets created. There is an email field called "incomingmessage" that I could use to fetch the email, but its returning partial email not the complete thing.