Yes, that what I meant by GNOME Apps, but that is also available from standard linux distro, there’s no phone specific app store for L5 yet. I’d prefer it not to exist in ideal world. The problem is to know what is mobile-ready (screen/power optimised) and what’s not.
Preinstalled apps will be there, and every component will be removeable. Some of them are better left alone though, like the on-screen keyboard, software manager, or the kernel
There’s a plan to create a store which will serve primarily PureOS, but I’m not aware of any plans to restrict it only to PureOS, I would actually be very surprised if it was. So in a way it’s like any other distro.
Determining what is mobile-reasy is a topic we’re constantly discussing in relation to the store, so you can be sure there will be some way to tell.
I’m not sure what Google Mobile Services is about, but if you can find what you need on a standard Linux distro, then you can also put it on the phone.
yes, you can remove stock apps (eg Geary) and install your preferred app (eg Evolution or even mutt). But it will be your responsibility to ensure those replaced apps are mobile-ready.
librem.one is just a service bundle providing services like chat (matrix/xmpp), mail, social (mastodon) and vpn. Each stock app will be ready to join them with little effort (just enter your credentials) but of course you can ignore librem.one default offering and use your own services (that’s what I’m planning to do anyway). So same as 1. - you can uninstall/replace the apps but make sure replacements run ok on mobile.
P.S. some apps (like Calls or Chats) provide essential phone services (calling, messaging) but still can be replaced if you know what you are doing. No one can stop you installing sailfishos or ubports phone stack and hook it up to wwan modem.