Obviously the one that "melted" was not metal, otherwise the lower melting point alloys would have gone first on the turbo, so if it melted it is either not up to the job/material wise.
If it were the afr/mapping causing it to get enough heat to melt something here, ther eould be many many more melted parts first.
A pre cooler is a good idea, not only for any water supply to turbo but also for the oil, as the oil soaks/transfers away at least as much heat as the water does.
The engine location doesn't help.
tabetha