You have multiple, only tangentially related questions that would be better served being posted separately. Answering one of them...
Any tutorials/docs on how to get Jitsi work in walkie-talkie mode are highly appreciated.
I just attempted to use this featured and determined that is works in the same way as the Skype PTT feature, which is to say it is just a glorified mute button, with inverse logic. It does nothing to address rapid call setup, which is also a necessary part of what has come to be considered a PTT service.
According to the Jitsi release announcement for version 2.2 (2013.04.30):
Adds “push to talk” functionality (quick unmute while pressing Ctrl+Shift+T).
So you place a call, click some mute button in the UI, then use the Ctrl+Shift+T shortcut to unmute when you want to talk. (Messages in the Jitsi mailing list archives suggest you can change the key combination used for this.)