What exactly is the problem with takeovers?
You can't guard yourself against change of heart. If alliance is rotten, it's only a matter of time.
I don't see why anything should be done about it. It only takes away Chew's precious time which could be used on many other things that actually need to be done.
Fact is that some people just ain't fit to lead an ally and failure is inevitable.
Do you think that people such as Azull, phantasm, even Eon, MRAlyon and darkraptor need such change? They know how to lead ally, how to pick people that are fit for their alliance, they know how to keep their members on the right track, they know how to workout things with members and ensure everybody has common goal.
All you need to do is follow the example of good leaders.
But... if you really want mechanical solution, hell I got one (or few), it's purrfect!
After invite is sent, receiver needs to confirm it. When confirmed by receiver, leader can see applicants loyalty points, leader also needs to confirm the accepted invite.
Accepting invite would lower the new member's loyalty to 0. (would sort of make sense lol)
In order to takeover leadership, after surpassing the loyalty number of current leader, there is a timer of 7 days for old leader to show up and accept the leadership of the person trying to take over or decline it. Upon declination that member gets loyalty dropped to zero. If time runs out, it's assumed the leader is inactive and that other person becomes legitimate leader, same thing if old leader agrees to this change in leadership.
Pip's idea
But really, I think all of that is pointless. Good leader won't have any problems regardless of system in place. You are just shielding the bad ones.