I work in NLP, my company has a sentiment analysis product. It's a very small part of what we do, and it's focused on a particular application. NLP itself has a very wide range of applications, some theoretical, some practical, some already in use all day long at very popular websites. Two months is a very short time for complex problems in NLP, believe me.
Regarding tech risk, I think you're taking your particular product niche and casting it to something unnecessarily broad. Polarity detection is just one possible task in NLP, which is itself a sub-domain of AI. It can be done reasonably well without using any real NLP techniques even (hence the crowded social media monitoring scene). That paragraph made it sound to me like you are stressing yourself out unnecessarily about 'solving AI'. It's sentiment analysis (which can be a challenge to get right), focus on that.
Regarding market risk, marketing new technology is a challenge for any new technology, and any given startup will have the same questions that must be asked and answered. The application of your idea may be marketable or it may not be, that has little bearing on the viability of AI/NLP. The best you can do is make sure the company has done due diligence and has a plan, hopefully with some market research to back it up.