Dear questioner,
Thank you for your important question.
May Allah, Most High, reward you for your desire to increase in knowledge.
Question One
Yes, Pre-sexual fluid (madhi) invalidates the ritual ablution (wudu). It is a condition to be upon ritual ablution when you intend to pray.
Always being upon ritual ablution is not an obligation but a recommended action in the sacred law. It has many benefits, one of them being that it protects one from the whisperings of the devil.
(Tahtawi, al-Hashiya)
Question Two
If the pre-sexual fluid is more than 5cm in diameter, it is obligatory to remove it. Any prayer performed with more than 5cm of heavy filth is invalid.
If the pre-sexual fluid is less than 5cm in diameter, it is not obligatory to remove it, and the prayer will be valid.
As for your wet skin touching the pre-sexual fluid, it could spread it. Hence, it is best to dry yourself completely before putting the same underpants back on. In your situation, It is also recommended to wear new or clean underpants.
Be sure not to give into misgivings or the whispers of the devil. If you have dried yourself completely, be certain and pray.
Please see:
And Allah alone knows best,
[Ustadh] Omar Popal
Checked and Approved by Shaykh Faraz Rabbani