I quite like
Theodora,
Teddy or
Thea would be my choice as nns.
Thora isn't my style, since I really like
Thor for a boy.
I don't think it sounds pretentious, especially not
Teddy.
Nyx - Nicks? Not my style, sounds like a nice name for a cat, though.
Katarina - I like
Katherine,
Katia, and
Kaia but I cannot warm up to
Katarina,
Katrina, etc. I think its the "reen" sound.
Mariah - the name itself is fine, but I associate it to closely with
Mariah Carey.
Philip -
Philip has unfortunately come to represent middle aged talk show hosts like McGraw & Donahue. If it was just Donahue I could tolerate it.
Alistair - okay. Alisdhair or something flufflier more so.
Alec - I prefer
Alexander nn
Alex,
Xander, or
Andy.