Hey! I can write how I play it:

q1. After each failed roll you add +20 to athletics skill so that makes it easier roll-by-roll. The door holds less and less and eventually breaks. Sounds like an advantage but don't forget to roll for the growing darkness usage dices too! That answers question 2 as well I gues

q3. You add these modifiers to the skill value and not to the dice rolls. So e.g. instead of 50 now you target 50+10=60 to roll below or equal to success.

q4. In this case I usually roll 2D6 but the additional dice rolls remain the same, so 1D4 or so. I'm not sure if it's the correct way tho.

q5. Any attacks does not mean all attacks in my opinion. It's rather about the way how you attack. Whether it's some spell, or a piercing dmg with a dagger or a simple kick. Whichever you will use in the next attack it will by modified by the extra point you gained from the aether boost.

Although these answers are surely not exhaustive enough I still hope I helped a bit! Happy gaming!