Unless you have a higher priced or accurate gun, buy high end bb's and take the time to sight it in properly, a red dot or any optics does not do much for the gun.
Seek the services of a good gunsmith
No, since it is of inferior quality because it is chinese, if the scope gets shot at, it could possibly crack, injuring your eye.
FN P90 is a CQB (Close Quarter Battle) weapon. The barrel is short (also in the in the airsoft version) so it will not be effective at long range. You don't need scope. You need a red dot sight. Get an Aimpoint M2 (the most common red dot sight for P90), Holosight 551, or reflex sight. Those will suit for function and fit for the look of your P90.
No, not at a far distance. You would have more luck shooting and compensating than anything else. They are fairly effective at close range.
In my experience with any guns, as long as the gun has a tactical rail, you can fit pretty much any scope on it. For example I put my high tech airsoft red dot sight on my cheap nerf gun. You can modify what the scope can fit on because there are allen wrench screws to adjust the width at the bottom on pretty much any scope unless its already attached to the gun.
Yes it does have a red dot in the middle of the scope.
Not a good idea. Leapers/UTG products are made for use on airsoft and paintball guns. They're not designed to withstand the shock produced by a shotgun being fired.
an electric sight is a "red dot" sight that uses a laser to project a red dot where the airsoft bbs will go when you sight it it.
Follow the instructions provided with the scope/mount. You need to make sure you have a scope with rings that will fit on a "Dovetail" scope base rail. They have 3 main bases; dovetail- mostly for air and pellet guns...some 22's also, Weaver style, and Piccatini rail systems.
Yes
There are no "good" scopes for that price. If you haven't the money to buy a decent scope, odds are you don't need one.
Never!