"2 different figures are floating around:
1) Twice the volume of Lake Michigan
- This is the amount of proven ice in the top metre of regolith around the polecaps"

That makes sense.

"2) Enough for a 500m deep planetwide ocean
- This assumes that the ice we see at the surface is effectively the tip of an "iceberg" that continues to a depth of 5-10 km"

This may not, though I'm sure the people who have made the assumption have a good reason for it. Why not 1 km or 30 km? Maybe gravity variations?

As with anything which brings new information, there may be more questions that answers.

A person could wonder as to why or how the ice formed at the pole if the place weren't warmer and wetter.

Would the atmosphere deposit sublimated water vapor at the poles?

If that much vapor, why significant amounts didn't disappear into the void? Or maybe it did, then why that much water? An ocean 500m deep around an entire planet seems like one whole lot of water.

If a significant amount came from comets, wouldn't that be deposited more evenly around the surface unless warmer, wetter?

Those aren't necessarily questions for you, but idle musings from someone who doesn't know anything about this stuff.

And, thanks for the reply.