"Mum" is a shortening of "mummy" (or similar) which almost certainly came before "mother" (or its ancestor words) as a word for one's primary female caregiver if not also birth giver, on account of it being baby-talk that ancient parents naturally took to be a name bestowed by the child.
In languages descended from Proto-Indo-European, the -t(h)er suffix is a familial grammatical particle that has long since ceased being productive, and remains frozen in all daughter languages. Speaking of which, the -ter of "daughter" is the same particle.
That "mum", at least phonetically, is also an abbreviation of "ma'am" is a coincidence caused by dropping so many sounds from the original "ma dame" that it reverts to, well, mumbling, which isn't far off baby talk, all things considered.
FWIW, there are places in the world where "Mam" is a name given to mothers by their children, which is also rooted in baby-talk and also has no connection to the other pronunciation of "ma'am".
I reckon I've been pretty lucky. The handful I've attended haven't been that bad.
The "worst" one, at least from my perspective, was probably a relative's where I was an usher and messed up something with the church seating. The guests sorted that one out themselves when they thought I wasn't looking. The wedding itself went without further problems, but that minor mess-up on my part will always stick with me.
The next "worst" was the one where the reception / after-party had a DJ who cranked the music volume another notch every 10 minutes. The venue had a literal decibel meter on the wall, and I think he had made it his goal to max that sucker out. I've been in clubs where the music is so loud you can't hear your own voice when you're talking (shouting) to someone else and this went well beyond that.
By contrast, the ceremony itself had been very demure and pleasant, in an English country manor house no less, and were it not for that DJ, it might have qualified as the best.
The best one was probably when I was a kid. I don't have any memories of the church ceremony, which has to mean I was bored out of my mind, but must have behaved myself and there were no problems of any sort. I vaguely remember the reception in a function room at a hotel and there was nothing of note there that I remember either, except exploring the hotel. Weather was good. Must have been perfect.