1. Post quality can not be determined mathematically. It is a subjective concept. Word count may return totally wrong figure as many bots are now posting garbage and copy-pasting long post by other users.
2. Yes. Trust matters. But, only +ve & -ve. From the perspective of signature campaign, a +ve trust account is not much valuable than a neutral trust account, given all other parameters are same.
A positive feedback from non default trust users isnt much valuable yes but what about positive feedback from default trust users? It should affect the price of the account IMO so if you can compute that too then it will give much better estimated price