What is the effective level for the evolutions manifested by Evolving armor?
As far as I know, there is no RAW on that as it's an extremely specific case. But I'd probably go with the Evolving armor's caster level since the armor is what's providing those evolutions using its magical power. Also, the spell that's used in its construction (Evolution Surge) can only be cast by a Summoner level 7 or higher.
Hence, you'd gain 10 resistance and could actually choose Improved Natural Armor twice. Having a +4 to natural armor probably isn't too overpowered compared to your actual, normal armor bonus.