1. Account - free account is valid until? is it 30 trail period or unlimited?
2. Can I link elements of same screen?
3. Can we do hover?
From what I understand, Adobe XD is in its Beta version and until there is an announcement to the contrary, the Beta versions can be downloaded and installed for free.
Link elements of same screen: I did not quite get your request, but does this link help? Allow interaction wires/links from objects inside a symbol – Adobe XD Feedback : Feature Requests & Bugs
As for the hover effect, this feature is under investigation Tap/Hover Effect – Adobe XD Feedback : Feature Requests & Bugs You can vote for this request in the link I provided.