1. Is 我在这个方面上比较弱 or 我在这个方面比较弱 correct when trying to say "I am weak on this aspect," or are the both correct?

people can understand you from both, yet they usually omitt something. like 我在这方面比较弱

2. In adressing people with phrases such as 你好，生日快乐，圣诞快乐，早晨好，etc., do you put them before the name or after the name?

e.g., Good Morning, Chen or Chen, Good Morning. /  Merry Chirstmas, Chen or Chen, Merry Chirstmas. 

it really dosen't matter.

3. What's the difference between 在。。。里 and 在。。。中？

inside vs in the middle of

e.g.,

i am in the middle of a meeting. (i'm having a meeting) -> 我在开会中

i am in the office. -> 我在办公室里.