Here is a link to a typical diving end
Diving Deep End
It shows 24', but I thought it was 24.5'. Anyway, the whole concept is that the sides and the deepest wall have steep slopes, so that if people dive into the pull and angle downward, if they hit their head, it would be more likely the force would deflect them off of the side rather than jamming their neck and possibly breaking their neck. Conversely, the slope to the shallow end must be gradual, so that if you dive flat, the same safety type thing happens. If the slope were really steep, it would be like diving into a vertical wall and bad things would happen.
A thought on a narrow FG pool is that if a kid dives flat into the pool, it is very easy to run into the wall if it is only 12' wide.
OSHA or someone like that developed the safe shape.
For some people, they say their diving board increased their home owners insurance rates. I checked with State Farm and for me they said it would not change my rates rates.
I considered a sport type pool, where both ends are shallow and it is a bit deeper in the middle, people will list it as 4-5-4, but I didn't feel that it met my needs as I like the deep. Plus I was worried that some kid would dive in no matter what was posted. And For me, pools my whole life have a deep end and a shallow end. So I would dive into any pool regardless of depth into the deep end. I never would have thought the deep end would be actually in the middle.
But moving into more of your questions/concerns. Vinyl liners typically will last 8-10 years if you buy the thicker 27mil one. If the pool is full sun all day, could be closer to 7. The walls will last indefinitely as far as I am aware. The steel walls could rust a bit and have to be cleaned up, but there are polymer walls that will never rust. Someone else might give you a better answer on this than me as I am currently a new owner of my first pool (though a lifelong swimmer).
One thing about vinyl is I ended up using a higher end vinyl PB guy. There are some local shops here in Cincinnati that don't focus on extras, they focus on selling a large number of pools. While they might build you a good product, they might not educate you on all that can be done with a pool. My PB really allowed me to do whatever I wanted. Of course, my budget didn't allow me to do everything I wanted, but I could have.
The traditional vinyl lined pool has a fiberglass insert for steps. Very rugged, very functional, but not elegant. Newer vinyl steps have liner over the steps and they look far more elegant. One thing that my builder did for me, that only cost maybe an extra $900 was to extend my top step to make a tanning ledge. So I have a staircase into my pool that is 9' long, but the top step is 6' feet wide, so it makes a nice tanning ledge. You can see a picture
here. This pool is a 20x40 oval, so I have a 15.5 x 20 shallow area after my 24.5 long deep end. Maybe not exactly big enough for a true volleyball game, but pretty big. If you went with a L shape shown
here you could end up with a diving deep end as well as a shallow end that is 16x30, which is a huge "sport" area. (I am not advocating for that particular company just that they have a picture that shows my point).
Vinyl is not for everyone, but it is cost friendly, and can be fancied up, though it will never look as elegant as a gunnite pool.