50%..... you might as well make it barb. So anyone gone after 2 weeks, even with someone else watching the account basically goes faithless in every province? That's just dumb. 5-10% is more than adequate if you really want THAT MUCH of a benefit when attacking them, especially with officers and tribe bonus. At 50%, they'd just waste troops doing anything, and support would all be useless too, if it went to the level you're looking for. If you're really not good enough to defeat them without getting THAT MUCH of a combat benefit, you really shouldn't even play. You say compromise, but you went straight to the extreme..?
5-10% combat bonus against the account after week/month of inactivity with NO LOG-INS from the original account,
OR
Account will be removed from it's tribe after 90 days without original player logging on, because co-op'd accounts can't join tribes without the original player.
Those 2 would be the MOST limitations that should really be put on Co-op'ing. If you really need more than that, then I feel bad for you, or TW should just not have co-op'ing whatsoever, because basically killing off the account after 2 weeks is a terrible plan to keep people actually playing, Because apparently, according to your plan, they'd be better off just going inactive with no one on the account, and waiting the MONTHS to barb, but still have 100% faith and morale on Defense.