Regarding the St. Helens, perhaps she
had escorts
.
Regarding the Eisen... in '73, the Midways were completely non-existent. Remember, they were Blair's project, and at this time Blair was only just starting his (post-Crisis) BW assignment, so he obvoui. Thus, if the Vesuvii are hellishly expensive, it's not a matter of saying "Oh, that's all right. We'll build a Midway instead". Obviously, a new ship is needed - one that can take a Vesuvius' place, but not its pricetag.
And yes, a heavy CV might be costly to develop... but do you really think that Tolwyn's pet Vesuvii were the only ship designs being considered? I think not. Any Navy thinking of building a new class will have more than one design in the contest for the money. Thus, we can be certain that this alternative heavy CV class
already existed (on paper, of course). Therefore, what it comes down to is the construction pricetags, and presumably Tolwyn's designed-to-be-invincible Vesuvius would have had a much higher price... and, embarrasingly enough, had just been proven not-so-invincible.