What is the ruling on the prostration of one whose fingers slip and separate despite their attempt to keep them together? And does that invalidate the prayer? And what is the ruling on one who hears sounds emanating from the anus without passing wind; does their ablution become void?

Bringing the fingers together during prostration (sujud) is a recommended posture. If one omits it, there is no sin upon them, especially if it is beyond their control. Placing some fingers on the ground is sufficient to fulfill the pillar. As for the sound from the anus without anything being emitted, it does not invalidate ablution (wudu') unless one is certain that something has exited.