Ah... you shaved off the corners so they wouldn't rub on the tires then? I was thinking you shaved off part of the face of the pad.
So these are small wheels as well? I assumed they were 26 inch mountain bike wheels. But yeah, if we're looking at 24 or 20 inch wheels then that is a horse of a different color. Long V brake pads WOULD rub on the ends when trying to use the long Thinline V brake pads.
Unless there's something odd with the frame where the stays are lopsided to allow for some aspect of the drive train then your two brake arms should be even on each side when they are sprung out evenly spaced from the rim's brake tracks. If you center and equalize the arms by hand and they stay in place and then pull the brake lever and one side stays in by the rim and doesn't retract than that arm is sticking. If you try to center and equalize the two arms and when you let go the arms spring to the position shown (or near to it) then that is a spring issue. One of the return springs is stronger than the other. If this is the case I'd say a full new set of arms is probably the cheapest cure given that your arrogant guy couldn't help out with a proper analysis and cure.
My analysis may be off too. I've only got what you've shown and told us to work with. But at the very least I'd suggest going to a new shop where the mechanic subscribes to that truism "the more I know the more I realize I don't know....".