1) Duplicating a transaction requires either right-clicking (CTRL-clicking for Mac) OR using the narrow button in the upper right-hand corner of the register to evoke the context menu containing the command to 'Duplicate Transaction'.
With a transaction highlighted, SHIFT-TAB moves focus to the narrow button. SPACEBAR opens the context menu. ARROW keys highlight the context menu's items. ENTER executes the highlighted command.
2) There is a setting in Preferences on the General tab 'Separate Tax Date for Transactions'. It has been my experience that once the option is checkmarked, you must restart Moneydance for the changes to take effect.
3) A Transactions report should provide this type of separation. Memorizing the report could provide easy access to it from the sidebar.
Attention IK support: Why is such important, current, and time-saving features such as keyboard shortcuts relegated to an "archive"? Especially when an italicized statement at the top of the page says "This is an archived document. Please refer to the more recent knowledge base articles on this page" only serves to confuse when clicking on the link brings one to nothing about keyboard shortcuts!