On 1) ACES-compliant EXR does have to do with AP0 primaries, but also includes metadata.
On 2) Rendering in ACEScg uses color primaries that are closer to actual devices - a little bigger than Rec2020, but AP0 is the target for File Outputs (and archive and interchange). When working completely within your own facility without sharing of files, ACEScg is sometime used for convenience but using the format in the name of the file to distinguish it from the ACES standard (putting ACEScg in EXR with the primaries specified - a device or AP1 - means it is not an ACES file. The ACES flag in a header should not be set.