Make keep and tower lords more powerful.
The underlying problem is not really the health of the keep lord, but rather the fact that WvW mainly consists of stacking the largest zerg.
Not giving rewards to multiple persons in Warhammer’s RvR made zergs unrewarding, and it often resulted in large zergs splitting up. This mechanism had flaws, and to large zergs were still a very common problem. Guild Wars 2 on the other hand, lacks any anti zerg mechanism. It rewards them. Everyone gets full rewards, regardless of how many players helped.
Regardless of players, I believe the keep lord should be made such that he can’t instantly be killed.
Instead of scaling the keep lord, the keep lord should be similar to some special boss fight, where there are mechanisms to ensure that the boss will atleast stay alive a certain period.
There are many options. Invulnerability, dissapearing for a fixed time and having to fight some mobs, or something more special. The keep lord could unleash a trap that freezes himself and everyone nearby with the upside that they can’t be damaged, and this trap will linger and freeze for one minute. This allows retribution from another world, as the zerg may be split in two between unfrozen and frozen, making them vulnerable to a smaller zerg.