I don't know why you are messing around with these half measures, you need to tear the whole house down and rebuild it from scratch.

Or just get some large joint filler, and fill and sand the holes. Paint when you are done.

All roofs are large by surface area and to make draining quicker and efficient they are usually sloped right to left. I think it's 1 inch drop for every ten feet. Also, houses are never level, there is always settling. Imaging pouring a pitcher of water on a flat table from the middle, it will eventually either drain off the edges or dry, but not quickly or efficiently. If the table is sloped it drains off quickly

They have to be sloped or the water won't drain properly. Standing water is bad

This is the way

Hopes and prayers only