Can the wetness from the urinary opening be continuous like vaginal wetness, and does it only refer to pre-ejaculatory fluid (wady)? And how can a woman differentiate between vaginal wetness and wetness from the urinary opening, given their proximity?

Most vaginal discharge (vaginal moisture) exits from the birth canal, and it is pure according to the majority of scholars. This is sufficient, and there is no need to research further unless proven otherwise. It is good to be cautious by washing whatever touches the body and clothes, as this avoids the difference of opinion of those who deem it impure absolutely. This discharge nullifies ablution absolutely, whether it exits from the birth canal or the urethra, and this is the opinion of the majority.