1.
In the following Plan structure, when I try to add a new plan under P-P1 or P-P2 is not possible as I don't have this option. In order to do this I have to set P-P1 or P-P2 as "top" and then add child plans. After I do that, I can set P-P1 or P-P2 under P and they will "carry" their child plans but in the case I edit them and change something the child plan is kicked out from its parent and becomes "top".

The reason I use this structure is because I need to give the availability of upgrading with pro-rating among various child plans and in case of cancelation or expiration to always have a bottom layer as P (registered) to fall.

P (exclusive)
- P-P1 (non exclusive)
-- P-P1-C1 (exclusive)
-- P-P1-C2 (exclusive)
- P-P2 (non exclusive)
-- P-P2-C1 (exclusive)
-- P-P2-C2 (exclusive)

2.
Under Plan->Options->Display there is "Hide Options if Plan is Not Selected" but it refers to Options-Options and not Options. Is it possible to hide Options (including Price options) in a plan?

I do not recommend forcing it and instead probably need to reconsider how you have your plans setup.

Have followed your advice and changed the structure to 1 level depth.

That's only for plan options. You cannot hide plan prices. There are no plans to support hiding plan prices.

That's ok

Thanks