Welcome to the world of TurnKey GNU/Linux.  Sorry to hear that you had some problems getting started.  Despite the name TurnKey, there is still a bit of learning curve for new users, especially for more complicated appliances like LXC.  I remember being exactly where you are when I started working with the LXC appliance several years ago.  Reading the thread above, I would say that you're getting up to speed faster than I did. :)

• Documentation is always the last thing to be written
• By the time documentation is written, the developer is so familiar with the operation that it's easy to miss the parts that are not obvious to a beginner

Thanks for being an involuntary guinea pig re: testing of the v15.0 LXC appliance.  Your experience should help us improve the documentation for v15.1.

I've issued a new git pull request that addresses the network issues you mentioned.
Update confconsole for v15.1 #25  They are now waiting for Jeremy's approval and releasing an updated Debian package for confconsole.

In the meantime, if you would like to test the changes in your environment, you can download new copies of:

/usr/lib/confconsole/ifutil.py
/usr/lib/confconsole/plugins.d/System_Settings/hostname.py

To run the test, first backup the original files above and your current /etc/network/interfaces file.  Next replace the files from the links above and restore the default /etc/network/interfaces which should look something like this

# UNCONFIGURED INTERFACES
# remove the above line if you edit this file

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet manual

auto br0
iface br0 inet dhcp
    bridge_ports eth0
    bridge_fd 0
    bridge_maxwait 0

auto natbr0
iface natbr0 inet static
    bridge_ports none
    bridge_fd 0
    bridge_maxwait 0
    address 192.168.121.1
    netmask 255.255.255.0

Then you should be able to use confconsole to switch between DHCP and static modes as well as change the hostname.