Hi Piotr,
Yes you can buy crypto with VISA/Mastercard or any PCI compliant card issuer.
The 5$ are provided in CRPT tokens if I am not mistaken (would not make sense otherwise, but I might be wrong) Both the referral and referred get the same. If I can find where it says in what value you receive the 5$ I will update the post and let you know. But as you read, you need to transact more than 50€ to be eligible for the 5$.
I think the "cash out" feature works with ANY Mastercard, being that a credit or debit card. It's another thing I want to test once I can.
This app will only work for Android and iOS. But can't be probably an Android of 10 years ago... no. And some low end phones it does not work too (but this general to almost all apps nowadays).