Hmmm... Not convinced. Not sure how his past convictions would be any leverage. The current charges could be. But he’s been charged anyway.
Maybe they thought this would be an embarrassment to Trump to be associated with Nader. But in my view its humiliating to Mueller that they would rely on him at all - and that they cited his name 100+ times in the report released in April when they knew at least 6 weeks earlier he would be charged for child porn.
If they had him under electronic surveillance, they may have known of the current problems. Or, they may simply have played the odds.