Interline is only available via Touchdown but staff travel on the route network is limited only by your own need to use it.
All bases are, technically, up for grabs: a new joiner will get a choice of 3 but if the company has an over-riding need to crew a new base the chance are that you will be sent there. Transfer elsewhere is via a bid system: if you bid to move, say from ALC to STN, and the bid is accepted you have to move, there is no turning it down. Stops speculative bidding.