Shorts would indicate that there is too much carbon in the pre-chambers and that the glow plugs are grounding there and burning out.
Definitely, if the relay is staying on too long the plugs will burn, these are the series loop type.
Find a better mechanic, these aren't that hard to figure out. However, I'd make sure he was 50 ish or older, a young guy may not have ever seen this type of starting system!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!