1)a)Yeah that's would be ok personally I think more emphasis on the day would be better but some country's use MM/DD and others use DD/MM so it might be confusing so like you said is probably better or if you can make the option to choose between those two would be even better.
b) if you mean that there might be duplicates between folders I think it's ok.
2)a) If xxx.com is on whitelist and tab url is xxx.com then dont swipe that tab. You could make it so if you whitelist xxx.com then all the subdomains from xxx.com will be also whitelisted or you could add a wildcard system so that if you whitelist xxx.com then only xxx.com whill be whitelisted but if you whitelist xxx.com/* then all the subdomains will be whitelisted this for example is usefull for pages that have videos on them i might not want the video page to be swiped so i can whitelist only xxx.com/player/* instead the whole xxx.com and it's subdomains.
protected tabs =/= pinned tabs. Protected tabs come from add-ons such as tab mix plus and tab utilities fixed. Protecting a tab means that it cannot be closed this prevents it from being accidentally closed and some other add-ons allow you to do more with protected tabs. If you implement the whitelist adding compatibility support to protected tabs wont be necessary.
Please make the whitelisting easy to access. If you can add an item to the tab menu something like
Tab Sweeper: -> whitelist xxx.com/* | whitelist xxx.com/currentsubdomain/* | whitelist xxx.com/currentsubdomain/* | Open whitelist
If you use the wildcard else just the first and the last items of the submenu on the tab menu will suffice.