TBB Alpha doesn't work with external/system Tor
TBB Alpha 6.5a4 unconditionally sets 'network.proxy.socks' to 'file://.../Browser/TorBrowser/Data/Tor/socks.socket' and 'network.proxy.socks_port' to 0 each time Tor Browser is started.
It doesn't matter if 'TOR_SKIP_LAUNCH=1' is set and it doesn't care about 'extensions.torbutton.use_nontor_proxy'.