2 - With a concurrent license setup, QTP obtains a license from the specified license server. That license periodically expires. QTP automatically keeps renewing the license as long as the QTP application is running. The QTP workstation must be able to reach the concurrent license server at all times while QTP is running. The exception to this is the use of Commuter concurrent licenses. More information about commuter licenses is available in the Sentinel documentation that comes with QTP.
3 & 4 - A QTP workstation can be connected through QTP to only one QC server at a time. Since the connection between QTP and QC requires that you install Add-ins and you can have only one version of those Add-ins on a machine, if you want to shift the connection between multiple QC systems then all the QC systems need to be at the same version and patch level.
4 - Connections through QTP or through the QC UI? What versions of QTP and QC are you working with?
(Opinions and information contained in this post are wholly my own and do not reflect the opinions of my employer.)