Sometimes I accidentally send a relative to a monastery or decide I don't really want that person working there. There should be a patch that lets you revoke monkhood.
While pretty much every monastic order to ever exist has had a sort of trial period, once you take your vows you can never leave under pain of excommunication. I'm unaware of any historical cases of rulers trying to fight that, though there probably is the odd one.
Becoming a monk should be permanent, as it was(is).
Sometimes I accidentally send a relative to a monastery or decide I don't really want that person working there. There should be a patch that lets you revoke monkhood.
I know of one case where someone in Iberia was pulled out of the monastery to be king to avoid a succession crisis. He got married, had a kid, then immediately retook the vows.
On the other hand, why do sons that have been ordered to take the vows still generate landless son prestige loss?
Yes you can. Convert to a non-Christian religion. Have the nun/monk also convert. Nun/Monk trait gone.Once you have taken the vows, you cannot go back.
There are several instance of people being forced into the vows and later being recalled or freed when circumstances changed. This was particularly prevalent in Byzantium Theodora and her sister Zoe came and went from a convent a number of times and Robert Guiscard during his attempted invasion of Byzantine was claimed to be doing it to restore Michael VII who had been forced to retire to a monastery.
The rules were somewhat akin to the divorce laws. The church technically didn't allow divorce but if the church officials wanted to or were being forced to you could find a loop hole.