1)
If it states it requires CEP 1 you need the CEP1 pack
https://neverwintervault.org/project/nwn1/hakpak/cep1-complete

As just user you don't need to worry about Compatbility of CEP 1 and CEP 2 just have both installed
Lot of changes occured from CEP 1 to 2 so newest CEP1 is backwards comptabile with all CEP1 and newest CEP2 is backwards comptabile with all CEP2

Hope that answers any questions.

2) PRC can actually work with ANY module including the OC.
The only exception is Preiumum modules that are encrypted so can't be modified.

PRC comes with tool to try to automate some of the conversion for modules to work with PRC.
For module to be able to use PRC races skills ect it needs to include the PRC .hak files and change some of the events modules have to register PRC events.

Which you could for example do manually by going in with toolset and going module properties included haks and adding the prc files and changing the TLK file to prc one. or the PRC/CEP merged TLK table (which is file containing custom text for HAKS)

But when you install PRC there is some tools it comes with to help modify modules to work with it... the main campaigns that come with the game work perfect with PRC and majority of modules you will find on vault do as well ones with CEP or without CEP.

You will run into some problems with some modules that use Project Q for example. (very rare) or require their own spefic tlk file that isn't CEP's.

There is problems you can run into with other things like custom experience systems and you want to use the PRC experience system instead but if you know what your doing you can patch it really simple.

So 90% of modules will work with the PRC patcher.
The other 10% can be manually made to work with PRC.

When module states PRC that just typically means its balanced around PRC
Though Endless Nights really isn't so balanced for PRC as it claims ... there is one extremely overpowered build with the settings that module uses.