This is so complex people study it for years and still don't get it 'right' for every situation.

Your hum can come from an amp that has problems, or long runs of mic cables of the wrong type, or faulty cables. You can even get hum picked up by the speaker leads, that then gets fed back into the amp.

In a building like a church, it was designed for a longer reverberation rate - 'echo time', i.e. it is assumed that you will speak slightly slower, so that the sound is not 'eating itself', but the echoes of one syllable die out before the next one hits. Throwing speakers into that situation can actually make things more inaudible. Ever been to a large showgrounds where you can hear the ring speakers sound arrive one after another? If you have just one lot of speaker (or one each side that same distance away), you can have one set of problems, having several rows of speakers long the side makes another nightmare, like the showgrounds I mentioned.

I know of no simple book that can help, sorry. You didn't say who 'set up the sound system'. If it was some rock group sound guy roadie self taught expert, just shoot yourself now, it'll be easier. :-0 The only thing they know is More Watts Man! It sounds like this happened if your ears hurt!

"The pastor is so over-amplified that she sounds tinny" - the sound is not balanced across the spectrum - tinny usually means that the higher frequencies are being over boosted.

'Avoiding feedback' - basically, you have to stop too much of the output getting back into the input. That is a black art in itself... :-) But the higher the output, the more chance of reflected sound from the speakers bouncing back to the mics, it doesn't really matter that the sound can be delayed, that in itself doesn't stop feedback. The 'howl' will usually be at a particular narrow frequency range - so a trick is to add a circuit that cuts out that frequency range, and that can stop the squeal. The hum, however is at about mains frequency, but has 'overtones' - 'harmonics', and they can be a nuisance.

I'm sorry to say this, but as a relatively inexperienced person, you may eventually end up with two choices, having to get a 'real professional' and a fat cheque nook, or the older people may just stop turning up.

Acoustical Engineering is the area of knowledge, but if you search on that, you will probably find lots of very advanced stuff that will probably not understand.