Please anyone feel free to correct me if I get anything wrong.

From my understanding, Apps dont change based on the profile they are installed in. They are the same exact app regardless of profile. Profiles just serve to separate apps for different users and further separate apps from each other with them being in thier own directories.

Not all apps that you install from the Google playstore require google play services. Some apps will run just fine without them if the devolper decided not to use them during development. If they did decide to use googleplay services, the app may function OK or completely break. Just depends on the app in question.

As far as I am aware, updates will happen separately for profiles. Most apps that are installed via googleplay will install automatically. Same goes for fdroid apps in most cases unless specified otherwise.

I’m not sure what you mean by your last question, fail how? Fail to open or update?

Also, profiles aren’t really necessary for grapheneos to separate google play services from other apps or whatnot because they are already sandboxed. I dont use them myself for this reason.