where I must put this file?
C:\Program Files\Nxt\ ?
Find your Nxt folder and put it in /html/ui/plugins relative to the base nxt folder
Is it possible to add (may be optional) timer till forge own block like it is in "classic" NRS?
Oh, you hardcoded first 20 forgers
It would be possible to do this but it's a little different that the original idea, might decide to add it in later.
Tes, it scans the top 80ish% of forging accounts because that is the most efficient way I've found tk do this, if you do all the accounts, then there's a good chance that pepole won't forge and its wrong just as much. In this scenario I'm mostly sure that given the opportunity, these accounts will forge the block.