The code is
For each 2 letters, take the letter between them in the keyboard. For letters at the end of the row in keyboard (e.g. M), the two letters will be the one next to it and the one on the other side of the row (NZ).
Not sure about the smaller letters, but they all seem to mean spaces or that the next letter is capitalized.
So the answer for eNZWRETETTUiXVGJETUOADRYNZLSADbUOBMgLSSFCBLSBMXVWR is:
Merry Christmas In Advance
It was a pretty nice riddle, but if i may, i would definitely suggest keeping the hints to yourself and only revealing them one by one every few hours, and only if people are having trouble solving it. The hints are very obvious, and immediately revealing them all makes solving the puzzle rather trivial.
Also regarding the puzzle itself,
Perhaps don't always encrypt a letter as its neighbors? In your "Hello" example, both 'L's were converted into 'KA' which is a pretty big tell. Perhaps you could encrypt one as KA and the other one as JS, either works as long as 'L' is the letter in the middle the two :)