So what's the question?

Sry haven't read the title.

You cannot reset kills. You can only reset kills score.
Btw. that has nothing to do with leaderboards but with the fact that sc doesn't let you modify kills.

store some values in DCs, use binary countoffs to copy the DC value into a leaderboard value (custom score).

you can have many many different things displayed with that method...

But can I run "custom score" more than once as different things?

I need Customs for Flags.

yes, u can.

You just copy other values into the current custom score with binary countoffs.
You can can copy that DC into custom score, if the mode DC is 1, copy another one into custom score, if mode DC is 2, ....

in brief you just would do:
if (mode =1)
{
current player's custom score = current player's flags DC;
}
if (mode =2)
{
current player's custom score = current player's geysir DC;
}

There's only one custom scoreboard.
You could use minerals or gas as additional display, create numbers on the minimap by placing buildings in a static grid, place mineral blocks which you can set to any value and give custom names or simply a display text output.
Theoretically it would even be possible to use the countdown timer as a display.
For example 112:21 meaning flags 1,2 and 5 are cleared, 3 and 4 are set

Minereals and countdown timer are also in use

It's not helpful pointing out 2 suggestions that don't work and ignoring the others

Use resource leaderboard? Gas? Player kills 1 sniper give 1 gas.. Display leaderboard for gas..

Yes, use the custom score for multiple leaderboards.

Make sure that your initial state is at 0 before you reset a leaderboard,
and then use binary cutoffs to cycle through and add up your custom score.
Store all the variables in death counters to call up when you need them.

It is of course helpful to keep a good list of your death counts and what states they are at.


Hugel

You can also use unused units and the Command leaderboard. E.g. burrow some drones and set Command Most Units: Drones as a leaderboard.