IMHO I think you simply being stupid diving deeper than 25 feet solo.
I solo dive myself and been in a number of uncomfortable situations diving solo, where everything that could had gone wrong did, but at 20' it was an easy shrug off and barely note worthy. Had I been at 40+ I'm sure I would have likely hurt myself badly.
Follow the 20 feet rule and you'll survive to love another day.
Unless it's your J O B, solo diving at 40+ feet is simply an unnecessary risk (like driving 90 on the freeway-- yea, you can do it... but should you).
Find a nice spot for shallow solo diving and enjoy yourself. Diving does not have to be dangerous.
Dying is for dummies, unless you are a risk taker-- then each to his own I guess.
Do you also suggest waiting an hour after eating before entering the water?
If twenty feet of water means the difference between an easy shrug off and getting hurt badly I would say you are not ready to solo dive anywhere. Probably that was your problem.
And, psst... This may come as a great shock but you're gonna die one day so... what does that make you again?
I think the OP framed the issue very well when she said:
" -- If a buddy was there, and did all he could, then we know that what could have been done was done. Otherwise, the torment is the thought that something could have been done, and wasn't. I don't want my family tortured with that."
I think that is a well meaning sentiment but I wonder how it translates into real life.
In the end I don't know how much more comfort there will be knowing I died of a MI at 100' beside my buddy or that I died of unknown causes while solo diving. I think the coulda woulda shoulda's are just phases of denial/bargaining along the grief pathway or a way we humans have of trying to inoculate ourselves from a similar fate. The incident and accident forum is populated by people trying to place a buffer between themselves and death. Which can be a good thing or a bad thing depending on how obsessed one becomes. The thing is, avoiding one form of death just opens the door for another form to enter.
For a different perspective:
From a Buddhist standpoint, solo diving can be seen as being extremely beneficial. The Tibetan tradition describes the state immediately after death as being a time of great confusion and fear (the Bardo). It is during this time, alone, that we reach out in desperation for our next rebirth. This seeking of identity is motivated by both our presence of mind at the time of death and our overall karma. Solo diving can help to train the mind to be calm while alone during somewhat similar times of uncertainty. If one can thus maintain this "mind" during their travel through the Bardo states, one has a far better chance of choosing a beneficial rebirth.
That's my justification and I'm sticking to it!
I see life as a continuum that does not end with death, it just transitions. Like a diver who trains for the next dive I train throughout this life for the one that's coming next.