Phone is displaying garbage lines and not detected by USB cable.

Depends on the issue with the motherboard. Some repair shops will replace bad chips on the boards for you if they can determine the damaged competent. This would keep your data in tack. The reason the service guy is saying that it would be lost is because they will not be fixing your board they would simply replace it with a new one. Your other option is to send it to a data recovery center where they have methods of retrieving your information. The rub is this; the repair shops that do that kind of work a very few and far between. The data recovery places are going to charge you an arm, both legs and your remaining thumb, so you are REALLY going to want that information.

What you are looking for is a repair shop that replaces IC chips on cell phone boards. You might get luck on here and finding someone with a mail in service that can take a look.

Good luck my friend.