I know its difficult to Not think about the ECG issue, as it niggles constantly on your mind, and Im sure one of the reasons you have posted here is to look for some sort of support. As the others say don't worry about it. Many pilots have abnormal ECGs, and I am one. I told CX about it years ago. The Good Dr at the time looked at my ECG and said we will never mention this again, and shoved the report in the back draw.
Does CX know that you have a class1 medical, or applied for one? Do you require a Class 1 medical before coming to HK for assessment? If the answer is NO, I personally would not openly report to Cathay your problem, let them find it out IF they come across it.
Now wither Cathay is a career move is another thread.
Good Luck, stay positive and don't loose sleep over your ECG there isn't much you can do about it at the moment.