About your question on how they will handle difficulty in BG3, they´re answered that in the Community update.

Will difficulty choices affect more than just enemy health and damage? e.g. increasing the DC on some rolls while exploring the world?
Yes there are many features planned for different difficulty levels, which we’ll go over in a future update - but EA isn’t launching with difficulty choices, as we prioritize everything you need to have an enjoyable experience.

If they are handling difficulty level adding more difficult encounters, not just adding hp and damage to pre-existing creatures the modders of the hypotetical 6-man-party mod would have their work easier if you have access to the tools that allow the game engine to change encounters depending on your difficulty (and if you can change it midgame) or much more difficult if you have to balance the encounters for every character more you add in every difficulty mode...

As always, a lot of "ifs" We still have to wait until we have the modding tools. That would be after the EA, in an unknown date, according to the Community update.