Regarding stat increasing items
 

For any stat increasing items such as rings, armour or whatnot, some stats are written as STR+13, some are (STR+13) while some are STR+7(+6). Can someone explain please? Why are they written like that? Can they be increased by us? Do they have the same effect as normally written numbers? (I'm talking about the stuff in brackets)

the ones in parenthesis are additional/'bonus' stats. those are all random, and can happen to accessories that are obtained as drops as well as quest rewards, but not to those that are obtained from KQ chests.

we can increase the stats given by accessories now (ever since that update around a month ago or so) through enhancement.

and yes, they have the same effect as normally written stat +'s.

Alright, so I just have to elrue them if I want?

just like normal weapon/shield/armor enhancements, they go all the way to +9. well, weapons can go up to +10 now but only the weapons so far. but basically, yep ;D