i've never had any issues with metal head gasket but i do feel for you mate, it sounds like you've done everything properly as well.
You are correct though if its not sealed this time then chances are it isn't going it.
unfortunately once its bolted down they can't be re-used and you know what there manufacturer is going to say "its down to poor installation"
If you take it all apart again i would double check the block and head to see how flat they really are, normally if there perfect you shoudln't get any leaks at all.
Ben10, blocks and heads are skimmed with a huge circular cutter which is a cutting blade, not a stone which is grinding. It can be done this way but that's not the "technically correct" machine for doing this, its an engineering machine for making flat surfaces but the proper engine building tool is alot more accurate and is a blade, Means you don't get imperfections in the surface as the stones heat up and expand, or get a slightly different profile from start of the job to the end as the stone wears slightly.
For MLS head gasket ideally you want a surface roughness of 50 to 60 microinch Ra range, this will give a perfect sealing finish. The other thing to check is that the head and block were decked correctly and haven't been done slightly on the piss which will make it hard for the gaskets to seal again.
Tim