For the missions I suggest that a caution message be issued before taking part, as once the mission is accepted then the player can't perform any activities until the mission is over. Although the player will still earn credits from their career path based on their rank, along with the small daily stat increase, they cannot do any career tasks.
One of the bonuses of this however is that since the player will be off-station, they'll be protected from PvP attacks until the mission is over; which could save them a few credits on costly extended stays in hotel rooms (especially if they are on higher level stations).
This is not intended to be a reliable way of getting credits and stat increases, but just something for players who will be unable to find the time to play the game for a while.
With a lot of serious penalties to benefits, I could get sorta behind this.
No XP (overall and career)
10% of normal training stats on originating station where mission starts
No bonds (even if VIP)
10% of salary
Maximum of 21 days.
Can't train intelligence
Otherwise, it's log in once every six months, and yay! Additionaly it penalizes active players and this would be a lot of implementation headaches in my opinion. While I don't PvP, you were correct, would totally break that mechanism in game.
Gonna have to go on the good idea, but not for this. I'll bring it up at next council meeting - I simply don't think it's feasible.