I’ll focus on 2.
The short answer is no, it does not poach the market. Let’s say you have 100 people read it via open beta. And let’s assume that, because they read it, they will decide not to buy it. (This is not accurate, but let’s just say it for sake of argument). And let’s say your HG is released for $2.99. You just lost the princely sum of $52.33. Seriously, that’s all you lost.
$2.99 x 100 x .175 = 52.325
And in reality, many beta testers here DO buy games upon release because they want to support authors and because it’s cool getting the “official” version of a product you helped test.
You’ll need to sell 1000s of copies of your HG to make decent money, so losing 100 sales just doesn’t hurt you that much, especially since those beta testers (at least some of them) probably supplied feedback to strengthen your gamebook.