I'll start off by answering myself. I think a screenshot before flagging is appropriate and adequate for records. If it's a serious enough violation that it's investigated the server ought to have a corresponding record of the deletion.
That's just my assumption imgflip is done right.
I believe ImgFlip leadership is aware of this right-wing extremism issue and does keep good enough records but can’t confirm that. I’ve always been told “screenshot, screenshot.” Sometimes I do, sometimes it slips my mind.
I don’t think not reporting it is ever the right call.
And as a practical matter I don’t think authorities will be interested unless and until one of these nutcases actually goes out and starts shooting up the place. I imagine the authorities’ hands are rather full right now.