With all the concerns in the OP, the many
questions, many calls for more research,
the speculations of disasters, the lack of
lots of solid, detailed information, etc.,
where the UN, etc. can be involved doing
much of anything on the ocean floor will
be blocked for decades.
But there is an alternative: Use the TIFO
method -- try it and find out.
So, pick some spots on the ocean floor.
It's a huge ocean with lots of candidate
spots.
At some of these spots, take a census of
what is there. Gather some worms, crabs,
shrimp, nodules, parts of hydrothermal
vents, etc. Admit that these are not
exhaustive or comprehensive but just
samples.
Then at some of these spots where took the
census, do the mining. Leave the other
spots as controls.
Then take another census. Compare the
mined spots with the controls. See what
the changes and upsides and
downsides are.
Since some of the candidate spots for
mining are in the waters of sovereign
countries and, thus, largely out of the
control of the US, likely some of this
TIFO work will be done and not blocked by
the UN, etc.
Maybe then we will begin to find out.