On your first question : like you, I very much hope that Larian doesn't enforce restrictions.
I would be fine, if not happy, if they indicated the standard domains of each deity. Or perhaps flagged a combination that doesn't really makes sense, lore-wise. For the players who don't know the lore but who nevertheless care about making something sensible. But the associations domain-deity shouldn't be forced. There will always be someone who knows all the lore and can come up with a character concept and backstory justifying an odd combination.
From what I saw in the DnD 5E rulebooks, each cleric chooses only one domain. And there are just a few of them (7 to be precise, in the Players' Handbook). Each deity has a couple of domains associated to them.
On your second question : I hope that they move the deity to a specific tag, which is independent of the class.
So instead of the current tag "Cleric of Ilmater", such a character would just have the tags "Cleric" and "Deity : Ilmater". A paladin would have "Paladin" and "Deity : Ilmater". From there, the same dialogue options should be proposed. That doesn't prevent Paladins from also having options specific to their oath, but so far as I don't think I have seen any dialogue option based on a sub-class/archetype.
On both your questions : no one here knows for sure. It's very possible that Larian doesn't know either.
