(1) 2017 and 2019 use the same data base structure. The only real change was how non base currencies, foreign currencies, are now handle, using a more effective approach.
(2) for most people, those who do not use non base currencies, there should be no issues. Having said that there should be no reason to go back to 2017. I have moved from 2015 to 2017 then onto 2019 without any issues.
(3) this is really your choice, I always run the latest preview build since I started using Moneydance back in 2015. Take a look at the text link on the preview page to see if the bug fixes or feature adds are important to you. https://infinitekind.com/preview
(4) I have a window 7 and a window 10 computer, have used 2017 on both, then upgraded to 2019 on both and have transferred the same data set back and forwards between the two computers for testing. The Moneydance’s data set does not care if a computer is running windows 7 or windows 10.