Was the test carried out with both EGR gas pipes removed on a hot system over the full temperature gradient?
Many of these failed EGR coolers either seal or the leak is drastically reduced when at ambient temperature.
The test should also be over a long period, say overnight to spot a small leak.
The mechanic who made the statement ‘there isn’t a problem but from time to time the van loses coolant in 100 miles but then might be OK for a thousand miles’ is probably trying to describe an intermittent fault. Let’s face it they just want to replace it as buggering about with a pressure test is an unknown that’s sometimes difficult to explain to a paying customer.
If I were you I would evaluate the milage on the EGR against the new EGR replacement cost and further investigation costs.
If you have more than say 40K miles or more on the EGR it’s probably going to be half full of soot and not performing well anyway.
I acquired an EGR from
@supernova with 101K miles on it with the intention to see how feasible they are to overhaul. At 101K miles the entire heat exchanger was 100% full of solid, hard carbon and the valve seized open to the point where it took 4x the standard force to even get it to move, no chance to close it at all.
I did manage to clean it after making a jig to use a long series drill to get a hole through the carbon on each tube so I could apply chemical cleaning - it isn’t work the mess and man hours!