How current leads by 90° in capacitor while charge on other plate is always opposite, should we not say it lead by 180°
In a capacitor, current is the derivative of voltage, which means that current is change rate of voltage. (exactly \$I=C*dU/dt\$)

If you differentiate a sine wave, you get another sine with 90deg phase lag (aka a cosine). 

So the 90deg thing has nothing to do with opposite charges.