Hi Ian, thank you for your questions. I guess they related to our GoodData Platform product and not GoodData Cloud, if so, then yes, this is the correct place to ask.
To answer your first question, the date filter is picking the data related to the date filter used in your insight, for example, if you created your insight with the "Date (created date)", then the range for "This year" will show the data corresponding for this date. It does not have to always work this way, I
suggest checking How the Dashboard Date Filter and Insight Date Filters Interact.
2. Setting a default date dimension in the "Date as" picker is not an option. They are arranged alphabetically, so you may try renaming them accordingly or using numbers, for example "1. Created Date", etc.