1- The signs of semen are three:
1. Comes out with a stream.
2. With desire.
3. Followed by the chill of the body.
If these three signs occur, the outside fluid is semen.
If one of the three signs is missing, it is not semen. It is clean water, either Mazi or wadi.
2- No. Wudhu is not required neither for Mazi nor for Wadi.
3- Wadi is thick fluid that comes out after urine.
4- No need to change or clean any of what you mentioned.