Greetings,
First off welcome to the forums. Good to hear you did the searching and I can understand your confusion. I'll try to do this as simply as possible!
1. I'm not quite sure which the better OS is, I myself have the latest version of Telestra and am quite pleased with the call, ringer and speaker phone volume.
2. Again I'm not sure if you will see a difference in volume between the two however as I stated I have had absolutely no issues with the latest Telestra.
3. The Zen theme is great for simplicity. It puts 5 icons of programs on your home screen plus a menu icon that will bring up a menu of all of the other programs on the BB. I have the 5 main programs I use on the home screen and bring up the menu for everything else.
4. Personally I do recommend the latest Telestra OS with the Zen them. It's nice and simple and with all of the shortcuts it makes for just all around a great setup.
Here are a few links to instructions on setting up your BB with Telestra and Zen. Hope this helps!
http://www.blackberryforums.com/ring...8700-only.html
http://www.blackberryforums.com/ring...-4-1-only.html