its impossible to police the no co-player idea. how would you differentiate between someone using their home pc, work pc, phone, and tablet?... i have 4 devices i can and do play from, some from mobile network, some from home network, some from work. it's all me, but could look like 3 or 4 different people.
An alternative might be having a daily time limit, so a bank of hours which counts down whilst you're actively playing. Or perhaps a daily click limit. This would create the level playing field, co-player or not. It would also make defending challenging - unless you allowed sitting of the account, and allow that to use up the time of the person doing the sitting...?
Something less experienced tribes often suffer with is poor leadership at the top, and the Duke being untouchable. Perhaps introducing a vote of no confidence which could be issued privately if a certain proportion of the tribe declare they have no confidence in the duke. The effect being that the Duke is demoted to council member, and all those with council rights (not just baron) are then eligible to be voted in as Duke. What they then do with their new power is up to them as they receive all the Duke powers, they just can't promote the duke that's been demoted for a time period (couple of weeks or something to prevent a viscous circle forming?)