To my understanding cohabitation alone isn't enough for a common law marriage. As long as you don't refer to her as your wife and your place is under your name, then you should be safe cohabiting as much as you want.

In theory yes. In practice finding that those things never happened requires a divorce court ruling against a woman, and if that were a realistic prospect we wouldn't be discussing this in the first place. If she manages to get you in that courtroom you've already lost.

If she's contributing to bills of any kind then you should claim that as "rent", so that way if she ever tries to claim your property then you can say she's just a tenant and evict her.

Until she makes false accusations of domestic violence. Then VAWA comes into play and overrides any laws that might protect the man. The legal situation around cohabitation is more variable than marriage but it's not a reasonable legal/financial risk for a man to take regardless. Frankly having kids with a woman has many of the same pitfalls but if it's a nonnegotiable then mitigating the risk is the name of the game and guy you're responding to has some very good advice in that regard.