I've seen two approaches:

A. Put down underlayment, liner - then fill the pond to settle the liner.
Drain the pond, then put in the rocks, gravel, etc.

B. Put down underlayment, liner, rocks and gravel - before filling the pond
for the first time.

Is plan B okay?

One other question . . . the 45 mil Firestone liner is supposed to last for
at least 20 years. How long have they been in use? Is it going to fall
apart at 20 years and one week, or will it likely last for 30 years? Does
anyone really know [I will have the liner completely covered by rock and/or
gravel]?

Are leaks inevitable, something I should plan on dealing with regularly, or
can I expect years of leak-free use?

Thanks