Also to add.  99% of this is about avoiding legal disputes in the first place.  Whether or not you intend to follow my advice, you'd be well advised to run whatever approach you do settle on by any contacts you have at ATI for their opinion.  It ultimately matters not who would win in court, unless you have the inclination and resources to see a lawsuit.  What matters is whether you're doing something that might cause ATI to feel aggrieved and which might incline them to take legal action.

EDIT: But don't worry too much if ATI is effectively defunct and you can't get a formal opinion.  An informal opinion is still better than nothing (particularly given unspecified legal complications with the IP) but I would say it's by no means essential, and I wouldn't worry unduly.