What is the ruling regarding the spot found on underwear after waking up from sleep, and is it impure, necessitating changing the clothes or sprinkling them with water? And is it permissible for a latecomer to join the Imam in the Maghrib prayer, then complete it and subsequently join the Imam in the 'Isha prayer, which was combined with Maghrib, without a new opening takbir?

That spot might be urine that has dripped, which is recognizable by its color and odor. If it is urine, then only the impure area should be washed without changing the garment. It might also be semen or prostatic fluid (madhy). The absence of a feeling of lust does not negate it being madhy or semen. It is advised not to hold urine back due to its health risks.

Regarding prayer: Catching one rak'ah of Maghrib with the imam and then completing the prayer makes it valid. Joining the congregational 'Isha prayer in the second rak'ah is permissible, but you must make the intention and say the opening takbir (takbirat al-ihram); otherwise, the prayer is invalid and must be repeated, because takbirat al-ihram is a pillar without which the prayer cannot be validly commenced.