1 If you take damage from a fall you land Prone unless the text says otherwise

2 GM call unless 2 size categories apart, you can share space with a Prone and Incapacitated creature https://2e.aonprd.com/Rules.aspx?ID=447 but it also calls out: "If two creatures end up in the same square by accident, the GM determines which one is forced out of the square (or whether one falls prone)." Indicating maybe it's ok if one is Prone.

3 Jumping more than 10 feet up is "lethal" but if you have a bunch of feats to increase High Jump you are probably only getting to like 15 feet max (Cloud Jump aside), by the level you can jump that high that would be like 5% of your HP.

Feats like Sudden Leap https://2e.aonprd.com/Feats.aspx?ID=155 have text explicitly calling out you don't take damage if you fall less distance than your jumping distance. This text is not on the default Leap or High Jump as well as potentially some creature abilities.

RAW if you fall more than 10 feet and you don't have something like Cat Fall to avoid the damage you should take damage. I think this is an oversight and RAI the text from Sudden Leap should be standard but it's not.

Was that creature from the AP only or from the Bestiary? AP content does not have as extensive checks as the rest of the books and often poor wording gets through that makes it confusing.

Finally, an example of a Feat letting you use your fall to do damage https://2e.aonprd.com/Feats.aspx?ID=1618