1) If you don’t see the urine or najasah and you don’t know it’s there then there is nothing to worry about. It’s best to check every time just in case
2) Yes if the water for sure is splashing from a najis area, you must wash wherever the water splashes
Avoid the water splash, or clean the toilet seat
3) If you see urine clean it. It doesn’t not affect your wudhu provided area being washed or wiped in wudhu is Tahir