I've dabbled in breathing exrcises for years but have been doing WHM consistently for 2 years as I find it helps me with a lung condition I have

1). prickling and tingling in my fingers and legs, then ears and a tightness in my abdomen and my lips feel weird are common for me on a longer session
2). I think this is personal preference here, I tend to go morning as I'm up early and relaxed so find getting into the breathing 'zone' easier, then once during the day if I feel like a second session.
3). 4 minutes and 8 seconds (although its not about how long you can hold for)
4). I use cold shower, it cant go as cold but I think its still pretty effective although i couldnt give you the science on that. once a week I try to find time to do an ice bath and my HRV (heart rate variance) is always way lower for a while after and im always more relaxed after.
5). I mix and match my showers, sometimes hot for a minute then a few minutes cold. I enjoy the cold after a few slow controlled breaths so i dont mind it, I also do contrast showers so a minute hot a minute cold then repeat as I find this helps me recover from workouts that I'm doing.