I have used this approach on three different phones, you need to make sure the + connection on the charging pad is soldered onto the connection on the USB port that is marked by the CN2 arrow on the main board.
I had one that showed the same issues you are describing here, taking the phone apart and resoldering the + connection solved that issue.
I have created a Guide describing the addition of wireless charging, it is in no way affecting the phones stability.
I was looking for the same solution, and I think it is possible, a closer look at the USB port from various spare parts suppliers, I was in doubt if mine would look the same as the one displayed here, after disassembling my phone, it turned out to be this one: http://bit.ly/1GR4SzP
And it is possible, when messuring connection points, it turns out CN2 on the bord is the +, and - you can fit onto the side of the USB connector itself.