what % remain should i recharge the battery is the best for maintaining the battery
I would not worry about this at all. Your battery will lose more capacity from aging (time) than from misuse. However, the manual does recommend storing the battery at between 50 and 70%. If you always use it for short trips, I would recommend riding with the battery close to full charge than closer to empty. Li Ion has a higher risk of damage when doing things at the extreme - i.e., discharging with the battery dead or charging with the battery full. The control unit on the bike won't let it damage itself, but you could theoretically help it by only riding with it in the middle to upper part of the SOC. Honestly this should not be a concern unless you plan on running it to 100k+ miles, and even then the calendar life would have a much bigger impact!
if i ride the bike using 6th gear all the time, even when starting the bike from all stop, will i gain longer riding distance?
No. You will draw higher current with it in a higher gear, and drawing more current is less efficient than drawing less current. The most efficient way to accelerate is to accelerate slowly and use every gear. Depending on conditions, the motor is most efficient between 4,500 and 5,000 RPMs. It is best to keep it spinning between those two numbers. For me, that means I never use 6th gear. However, I find I accelerate much more quickly when banging through the gears because it is fun, which probably offsets any efficiency savings...
what actually is the different between normal and sport mode?
Put in on a dyno and tell us
The display will give a general idea on what the bike is doing, and you can decide for yourself what to believe.
