Why are you asking us? We don't have the copyright and we cannot license it to you.
You've already been in contact with the creator, these questions are better suited for him.
Personally, I would think both your scenarios are okay since no-one is forced to pay you for the use. But, as I said, I'm not the copyright holder. You need to either have a lawyer look over the licence or get a very explicit release from the author stating that what you want to do is okay.
Okay, based on your edit.
Here is what my employer would most likely do (a company deadly serious about IP issues).
They would evaluate the relative costs of getting into trouble misusing the program (getting their considerable legal department to evaluate the licence) against that of just clean-rooming the program. They would choose the cheaper option and go with that.
Now here's what I would do, not having my own personal Nazgul of lawyers.
If I couldn't contact the author, then he's probably a one-man show - he'll be able to afford legal representation about as well as I can so will be unlikely to push that hard, in a legal sense.
I would argue that my application is indeed freeware since I'm not charging anything for its use. If a customer chooses to ask for extras (in-app purchase) or make a donation, that money transfer is not tied to the acquisition of my freeware at all.
Of course, if your freeware is near-useless without an in-app purchase, it could be argued that the connection is there between money and product transfers.
But I think I would be quite safe going the donation route. There is an absolute unbundling of the two events (product transfer and money transfer) there and one does not require the other.
Some people have even just asked for donations without a reciprocal arrangement (Save Karyn), although I wouldn't give money to some clown who'd already proven themselves inept at managing it.
Standard disclaimers apply: I am not a lawyer, I am not your lawyer, I don't even look like a lawyer from a hundred yards away.