Thanks Steve,

... and you're right about the pressure at 4000m - my mistake. 5000psi is what we're shooting for but customer has asked for future potential down to 4000m.

You're gracious to suggest I have all the answers. Sadly for me I don't. For example, the kinds of things I can imagine that might be said by experienced individuals go something like this (your comments being a good example):

"Never, ever use blah blah blah types of components. Our experience is that they quickly fail at very low pressures."
"Try to make your design using SMT. Thru hole technology has these known problems at high presures... Hybrid is even better, for these reasons..."
"Use this fluid instead of that fluid for immersion. This family of fluids adversely reacts chemically with certain components."
"Use this compound instead of that compound for potting for these reasons..."
"Always/ never do this or that because..."

Thanks again for your suggestions and help.
Best regards,
Steve C.