Hi,
Glad to hear that u get ur iphone, for those fake casing, is really hard to say, coz we really dunno what kind of material is that.
I got some of my buyer do try the fake case and scratch the back, maybe is becoz of the casing or maybe is caused by the dust.
Talking about the casing will make ur iphone back turn yellowish, i think this possibility is quite low, unless the casing melt hahaha. And i dun think it can trap the heat inside lah, coz still got hole for u to plug ur headphone, charger, volume button, etc, so many hole to release the heat

.
Since u oledi bought it, juz keep away from the water (When u yum cha with friend, u put ur iphone on the table w/o realizing there is water on the table), take out ur casing to clear the dust.
This is my personal opinion, hope can help u.
Cheer
Xavinlyn
thanks for ur answer and advice. I do want to get a proper casing for sure [in my mind now,neo hybrid with yellow frame
] but not atm bcos kinda broke after bought the phone by cash (afraid to do installment, eventho i have a CC). Maybe i'll try too get it by next month.
Anyway, u have any opinion about neo hybrid? I think it look good and fully protect the chrome part too rite? Pls advice