I don't think any comparison is implied.
I would read quam Karus as "(just) like Karus", as in the translation you give.
L&S says that quam can be used with an elliptic tam (II.A.2).
In this reading quam does not seem to differ much from sicut.
That is, zelotypos quam Karus haberis ≈ tam zelotypos quam Karus haberis ≈ zelotypos haberis sicut Karus.
I am not sure if sic and quam should be parsed as a pair in the spirit of II.A.9 in L&S.
I would rather consider them separate, with an implicit tam to go with quam.
I got the impression that zelotypus is typically spelled with -us.
The word seems to have a Greek origin, so the Grecism in the nominative is understandable.
Choosing between the nominative endings has no effect on whether all letters of the alphabet are covered or not.