Im no expert by any means, but will have a crack at some of your questions.
1. Cant see any reason to leave the steam tap open while warming up your machine from cold. In particular if you have a single boiler, dual purpose machine such as the Silvia or Lelit, this could be likely to drain your boiler and kill the element (not good).
2. I assume you are referring to the volume and times for a shot of espresso. General guide is ~25-30ml over 25-30 sec for a single basket, or ~50-60ml over 25-30 sec for double basket (depending what book you read!). Dont think the size of basket should effect the volume or time of shot (happy to be told otherwise though)
3. Single 58mm baskets seem to start from ~7g upwards, Doubles from ~14g upwards to triple shot size, but vary quite a bit on what basket you are using
4. Might need to refer that one to the Politics Snobs website ;)