I have a question about War Weariness. Will War Weariness be worse if you have war with more opponents than only one opponent?
Only if you're losing those wars!
Wars declared against you actually give you 'War Happiness', which will last until you start suffering attacks/losses from the Civ which declared on you. At that point, you start incurring War-Weariness points, which will eventually cancel out that initial happiness, and then (under governments which have War-Weariness) start making people unhappy. And the longer that war goes on, and the more losses you take, the more unhappy they will get.
Conversely, wars which you start give you no initial 'bonus' happiness, so (under Republic, Feudalism, or Democracy) you have no grace period before your citizens start complaining about the war ("All we are sayin', is give peace a chance").
But WW is counted per civ, so your citizens may be unhappy as a result of a prolonged war (including unit/town loss) with CivA, while also being happy about a new war declared against you recently by CivB. In that situation, by making Peace with CivA, you would make your citizens happier; but also simultaneously signing peace with CivB would make them
less happy.