For the things you see twice: This is because you display them both in your layout and you have also published the corresponding module. Display one of them, not both."stop sales" is match easier than before. In v3.3 you had to set stop sales day by day. Now you can set a whole season as "stop sales" by setting room's vacancy to zero. In v3.3 stop sales was controlled by the "special availability" which removed in v3.5. This is 1 of the 2 things that can not be transfered to v3.5 and the updater clearly says that you will lose special availability during update and you will have to re-enter these days by using the "seasons price".Why the date format "YYYY-mm-dd" is wrong? This is the main part of the global standard date format (YYYY-mm-dd HH:ii:ss)Which is the problem with the "Destinations" module? It says that it is compatible to v3.5+ only."Offlijne credit card" is included in v3.5, you dont have to download the payment gateway.
If you dont use vacancy then rooms are always available.Go to modules, click new, add the text you like in the editor's text area and assign your new user module on position "user1".You can set the date format in frontend, in backend it is always YYYY-MM-DDThe info if it is installed or not is not always available and depends on the type of extension. It is complicated to explain you why from here and it is not important.From what you have written there is no bug report.