Now with that said, some non-official legal opinion. So you had a company previous to this incident which had IP and employees? And a co-founder left, built the same exact company, with the same exact IP, and same exact employees? And then that co-founder got into techstars? And then that co-founder secured several million dollars in funding for said new company? And now you would like to sue this co-founder?
Maybe you see where I am going here. This is not exactly an open and shut case. Usually if a co-founder (or anyone else from within an organization for that matter) suddenly leaves with a substantial amount of IP and employees this raises the legal red flag right away. Yet you have come AFTER the company moved forward, received seed funding, received a series A round, and is now running at least presumably more successfully than it was before. That is not a good legal position to be in for you.
Second, although I have no idea what techstars seed round due diligence looks like, I am absolutely sure that the Series A round had a rather large legal due diligence process which included checking on the IP and making sure it was more or less clean. The start up world may be the wild west of funding but investors don't just handle over millions of dollars and hope for the best. This is not to say that a company can't lie, they can, some might, and often they do get caught during due diligence.
Sorry, the whole story sounds a bit fishy. If you are in fact in a small group of people who were "left behind" this could be entirely legal and even a viable business strategy. I have seen companies where a reverse merger acquisition has been made to move profitable employees and IP while leaving behind substantial debt and less profitable employees. I wouldn't call it ethical, but it is viable and certainly legal in some cases. Every situation is different though. Consult an attorney (or 3, because legal opinions are just that, opinion, until a ruling or a settlement is made), see what your options are, and move on.
In the end, all this depends, and IANAL but if the IP was developed before the company was legally formed and never properly assigned to the company then it might be a little tougher to go after him if he originally created it. At the same time, if the company paid to develop features etc and was formed legally, then there could be a claim, again it is likely messy.
You can absolutely find some good attorneys to at least sit with you in consultation and figure out where you stand. Honestly, even if it costs you 2-4 hours of their time that would be worth it.
In short: get to a lawyer. Search for grellas's posts about who to talk to cheap.
What is/was your role in the original company?
What was the cofounder's role in the original company?
How was IP assigned in the original company?
Why is the "legal route" not an option?
Did this co-founder sign non-compete and confidentiality and IP assignment agreements?