The prevention effect doesn’t affect damage dealt to a planeswalker you control (such as combat damage).

The prevention effect prevents 1 damage from each source an opponent controls each time that source would deal damage to you. It prevents damage of any kind, not just combat damage.

The last ability won’t trigger at all unless, as an end step starts, you have already been dealt 5 or more damage that turn. It doesn’t matter whether any of it was combat damage or not, nor does it matter who controlled the sources of that damage. (In other words, it will count damage dealt to you by sources you control.)

The effects from more than one of these schemes are cumulative.