MAD is less a strategy than a reality. As long as each side has weapons that can't be credibly destroyed in a first strike, you have MAD, whether theorists explicitly call for it or not.
Though of course submarines make this easier to achieve in practice.
I might be wrong about this though, perhaps there were significant differences between Brodie's 1946 theory and later MAD developments.
> As long as each side has weapons that can't be credibly destroyed in a first strike, you have MAD
With the significant qualifier that you need enough weapons to survive to completely destroy the enemy.
> MAD is less a strategy than a reality
I'm pretty sure that's incorrect. It was and is a specific strategy and implementing it was the reason for the ABM treaty and others - defensive weapons would make destruction less "assured". See the source I linked above.
People do of course adopt it as a strategy as well. But if effective defense tech existed I don't think the strategy would hold. The US abandoned the ABM treaty even without such technology.
As for your first point, it's true that MAD didn't really conclusively come into force until submarines. But there were efforts before then to maintain a second strike capability, such as keeping a certain percentage of bombers in the air, preparing them for fast takeoff, etc
Maybe not 100% assured second strike, but the basic idea was the same
Mutually Assured Destruction is not the same as Brodie's Nuclear Deterrence AFAIK (which admittedly isn't much). In 1946, neither side could come close to assuring destruction of the other; the Soviets didn't have any atomic bombs until 1949, the U.S. didn't have the H-bomb until 1954, and of course neither had ICBMs, The best production rocket was probably the V-2.