This one is a little bit more difficult to guess. If they stick with DnD rules, you can' just add in additional damage on creatures or additional resistance. They *may* have to fine tune each encounter to figure out the appropriate number of adds and at what CR they would have to be. I'd say less than 40% but this is a guess
Yes you can? This is literally how every DnD and DnD-like (Pathfinder, kotor) do difficulty options. Baldur's Gate have the standard difficulty being fairly below core rules. Owlcat's Pathfinder games have rules like enemies only doing half extra crit damage instead of 100%.
I'd expect the chance of difficulty settings to be near 100%. There's literally no reason to not have them. It's standard in video games to have at least 2-3 difficulty levels to choose from.
