Yes, of course.

I don’t know if I follow - you can use a SIM card from the carrier of your choice, provided the phone has the cellular band support. In the earlier days of Graphene CDMA had some issues (Version) but I do not know if thats the case still.

FAQ is here if it helps - https://grapheneos.org/fa

And you can ask questions through their community links found here (Matrix, IRC), it’s pretty active - https://grapheneos.org/contact

yes. you can use any sim card.it has been working well for me.