I cannot see how this would overall be a good thing. I would completely support a change that a member joining after war is declared simply cannot click the war button (EDIT: or attack/back any war run) which would limit the Merc issue. That one I do see as an unfair advantage. The other is just part of the game, people progress at different speeds and if a gold team can retain a 400 they must be doing something right.
Not sure what the effects of this would be, but what if the backing system was simply rearranged such that the original attacker gets two attacks, then a backer can attack, then original attacker, then all final backing attacks? It removes the need for specific tier based control, rewards lead dragons rather than brute force solo dragon, places a bit of control on the variation between the first and second dragons, and allows for better rage management/control (you know exactly how much rage the second dragon will need, and that means you can actually perhaps use a rage or two before swapping). It might even help with miscommunication- your teammate might forget to shoot down a red mage unaware that you need it to be killed. If implemented, I feel like this would only be effective in wars (possibly PVP?). The flame system would also be retained- first two dragons for five flames, next two for four, etc.
Bigger players would still win their own attacks without penalty, while smaller players will hit within their own range, but with two dragons, rather than one (again rewarding set up and strategy). Also, the penalty for having a bigger player back exists, but is not too steep- it only happens if you can’t beat the base with two dragons on your own, and only at the cost of one flame. This means your team can still hit that enormous sandbagger given that you have the resources/dragons on the team to hit that player, simply for four flames. Finally, it doesn’t change the number of total dragons in any given war raid, which means a three flame is still a three flame, and a four flame is (probably?) still a four flame. In this system, your larger players aren’t rendered useless- if you and your team is active enough, you may still be able to hit those higher levels for four flames, even if you don’t have backers of that size (introduces strategies of two lead dragons, preparing the base and follow dragons in more creative and diverse ways).
Honestly this might be a terrible idea but y’all know to call out a bad idea anyway haha