I think してて is していて, but I can't figure out what くるんて could mean.
Removing the slang it's read as
あ、えり足 は くるん と していて かわいい
hairline at nape of neck; nape of neck
くるん seems to be related to くるくる which means, among others
- whirling; revolving; spinning; going round and roun
- coiling around; winding around; rolling around
くるん seems to mean that the hair is curled only once, instead of repeatedly. You can see an example here:
In informal Japanese, sometimes 'giongo' (onomatopoeic words) are changed to give a slightly different effect. In your example, くるくる (definition) has become くるん and としていて has become てしてて. You see it in other giongo words sometimes. For example, つるつる/つるん(と), ぷるぷる/ぷるん(と), etc. I think it is more common with words that have the る sound. I'm sure others can provide more examples.