There is no simple answer to these questions, only contextual ones.
The evidence is generally that high-intensity exercise reduced chronic stress, that's pretty clear. But if someone is already very unhealthy, has heart problems, knee injuries, weak bones, etc, then they probably shouldn't do the typical crossfit workout.
An important principle to remember, as important as that negative, anxious chronic stress cases causes poor health, is that poor health impairs the ability to handle such situations. Inflammation, various nutrient deficiencies, and all that stems from that greatly hinders our ability to cope with things and once you we get someone eating properly and doing some exercise you we will improve their ability to cope with stress which improves their health and started the upward spiral. Once that happens then it might be a good idea to start IFing. Intermittent fasting is great but it is obvious from listening to newbies with metabolic syndrome that it is more a detriment than a positive before they get their health under control. Same goes for fasted exercise usually.
Cold showers is entirely personal. No way am I going to -warm up- to that idea :)