Returning PTC sites, would still invest on them?

Probably not. The reason they come back might only be becouse they want more money but without any work from their side. They open, do a little work so it shall look like they are serious (like paying), people starts to belive its a good site again and pay for Premium memberships, refferals and advertsing and in the begninning everybody gets their refferals, premium and advertsing but when many have bought it they close again and take all the money themself. So I would wait a while, if they are serious they will be there in 2-3 months and then you can invest.

Must wait to see some new proofs. For premium maybe ... (but must not to high) For ref, I recommend don't invest. Cause site will have hugh inactive waiting for you to buy

It would depend on the site and how well I had done with them before. I do have one site that I know that I would like to see open back up for business as I liked it a lot. However, for now I will stick with what I am doing.