well, not all this is true. the first part pretty much but :
the futures have the same volatility. The leverage makes the difference in $ at risk. but it works both ways (against and for you).
The signals are mostly the same for QQQ and NQ. However NQ is leading and commissions are much lower making you profitable before you will be with QQQ (the leverage helps there).
mini manipulated ?
well, if they are so is QQQ since it tracks precisely the mini.
NQ is not more difficult to trade, although I don't know what you mean by that. You are as fast getting in or out. the leverage puts more $ at risk but on the other hand you use less $ in your account.
some futures brokers are as stable as equity brokers even though not insured.