A384
Evolutions are not "locked in". Here's the key part:
"These abilities, called evolutions, can be changed whenever the summoner gains a new level, but they are otherwise set."
Whenever you level-up you effectively re-buy ALL of your Evolutions. So if you decide you now want the Large Evolution, then you must pay double for any STR/CON increases you also want. If you now can't afford them, you can't get them, and must spend the points elsewhere.
It's the same as if you had spend points on Tail and Sting, and then no longer wanted the Tail. Because you NEED a Tail to take Sting, and you got rid of the Tail, you must now ditch the Sting.

I suppose there's a slightly weird case of using Evolution Surge to grant the Large Evolution. But in that case I'd say it would only apply to any "new" Ability Increases.

A385
Claw Blades are a weapon attack:
"The blades grant the wearer ...and change the weapon type from a natural weapon to a light slashing weapon."
For all (most) intents and purposes, you no longer have a Natural Attack (with that hand) when using Claw Blades.