You probably can't condense your actions. It would require multiple components to notify each other and work together. At best you can set up an Autohotkey script which would press multiple keyboard shortcuts for you.
You can probably set up an autoplaylist or just a search pattern in general which would find duplicates. You can for example check if the first value in %date% equals to the second value:
"$if($stricmp($meta(date,0),$meta(date,1)),1,0)" IS 1
As I recall you can force some pseudo-support for string operations on multivalue fields. It's basically just using a function such as $stripprefix() on $meta(field,0), then on $meta(field,1), then 2, 3... etc (assuming your field contains 1,2,3,4... values). Depending on what you actually want to do with them, that might be sufficient for some.
Take a look at foo_dynfil about your virtual tags idea. However if you just want to essentially create a 'catch all' tag, it would be probably easier to set up remappings in the same fashion as the ALBUM ARTIST->ARTIST->COMPOSER->PERFORMER lookup works by default (it checks the first field, if that's empty, checks the next, until a field with a value present is found, which is then returned). For that the settings are in File/Advanced/Display/Autocomplete fields. You can achieve the same with titleformatting as well. For example:
$if2(%primary tag%,%secondary tag%)
The code above will return the PRIMARY TAG if it's present, otherwise SECONDARY TAG is returned (assuming that is not missing as well.)