Concerns about leveling, skipping layers of hell, and ending

I am currently trying to decide which campaign to run next and CoA stood out to me for the cool theme and high level. Though I do have some concerns that you might have a solution for:

There is only a range of levels for each layer, if I assume a level up after each layer this roughly works out. Whilst this could be a solution, it is entirely possible that the PCs have no business on that layer of hell due to no lost souls being located there and no patron objective being located there. How then best to make it natural for them to visit all the layers? Would a good solution be to have each PC lose their own soul and have a lost soul they have to recover? This does feel a bit too artificial imo and I would also have to spread out the exact phylacteries across the all layers of hell.

As written there is no real final battle, this could be an anti-climatic ending for some players and even though Asmodeus has a stat block, I do not see him losing to a couple of level 20 PCs. Is there anything that can be done to have a "final showdown" of some sorts?

Thank you for any advice given.