The Sachs seals are pretty reliable; you don't often hear of them leaking. The ohlins tend to give a bit more.
Your expert shouldn't be saying the internals 'may' be worn out. He has had it apart. Any wear or damage to the seal head/shaft/shock body should easily be picked up upon inspection.. and part of his job should be inspecting these items.
It's possible the new seal is getting damaged during assembly. The lip on the shaft is very sharp and unless if you dull it and carefully fit the sealhead (or use a tool) it's easy to damage things.
I'd be asking exactly what was replaced? Just the xring? The sealhead? The bushing? Was there any wearing of the anodizing of the shock body? etc. Now that you have a replacement you could perhaps have him inspect the following at his expense given that you've already had him have 3 failed attempts. Most legit tuners would be as equally concerned that their workman ship has failed.
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