I know some domains change your darkfire to various elemental types. I suggest that ooze domain alter it as acid. It makes sense and would save on finding a DM to change this over.
I think this is already the case if ooze if the is the first domain you picked - otherwise would be cool and makes sense yeah.
From my experience in ooze clerics, always have to ask a DM, but yes would make sense, and fit the ooze domain. Could apply this for different elements to several domains really.
Not sure what order I picked them in but it didn't change my element type sadly.
No you still need to ask a DM. I believe the reasoning they wouldnt add this is two actually 1) they want to give players a choice. Maybe they dont want a custom element for their deity. 2) since a player can just ask for it its really an unecessary effort to script this in