As said, I think it would be abnormal if you didn't have such conflicting feelings. You are torn between knowing that there was really no other way and that the situation, your mum and dad's health and safety required, the section, but at the same time you are torn because it's your dad who has been "taken away" to a mental health ward.
If it helps, my mum and I had the same thing when my dad was sectioned. On the one hand, there was all the trauma of the process (which actually went very smoothly), the knowledge that he had effectively been what people might term as "locked up in a mental health hospital" and the guilt that a lot of that was because we could no longer cope, was it because we were inadequate?
But on th eother, there was also the knowledge that we weren't inadequate, deep down we knew we could no longer cope, another week and my mum would have been in her own mental health ward with a nervous breakdown. And yes, there was also a tremendous sense of relief that the next day, we did not have to wake up to a bed soaked in urine, I didn't have to sit with my fingers in my ears to try to block out the sounds of dad shouting at mum whilst she got him dressed, and we did not face the prospect of having the entire day spent listening to rants and raves about what "those people next door" had got up to (in his latest paranoid fantasy). Both of us thought we would not be able to sleep, but in actuality, spent about a month sleeping 14-16 hours a night because the mental torture had been removed and our minds and bodies could finally try to repair the damage and exhaustion.
My dad died not long after being sectioned, from complications arising from fractured hip repair surgery following a fall in hospital (we were not surprised - as he had been starving himself and smoking eve rmore heavily, his health was very frail at that point) so we shall neve rknow what might have happened. I had already told the authorities that on no account was dad returning home, we would lock the doors! It had reached that point. The Ward Sister after looking after him for a few days agreed and said "you've been trying to cope for far too long" and yet curiously he was less challenging in hospital, possibly due to being away from the "evil neighbours"
Sectioing is never done lightly, and it requires either the Nearest Relative or a specialised social worker to apply for the order, and two doctors must agree.
A section 2 is called assessment, and the vast majority of people are initially sectioned under a 2 (there are others, but they are for emergencies). A section 2 lasts a maximum of 28 days, but can be shorter, during this time the person cannot leave hospital or refuse most forms of treatment.
If necessary, it can be migrated to a section 3 at the end of 28 days, a 3 lasts up to six months and can be renwed. Again, two doctors must agree to a 3.
WHoever is Nearest Relative (this would be his wife) will get some leaflets and a letter detailing what has happened, what can happen, and your dad's and mum's rights in the post within a few days.