It would of course be super nice if Larian were to confirm that they will invest effort into ensuring that people who buy the game on GOG are treated as second class citizens.
1- Not sure why supporting the Workshop implementation (something that they did for most of their recent titles) would mean treating you unfairly.
2- Don't forget that you chose your own ghetto. No one prevented you from buying on Steam if you cared about this type of feature.
1) Depends on whether mods end up divided between those who can use Steam to access them and those who cannot. If you're going to sell the same product in two different stores, it really should be the same product, and if mods become part of the sales pitch then the GOG version of the game shouldn't be dramatically inferior in that regard. Same if they decide to sell an Epic version. It's just not well mannered.
3) The more you divide the mod base and the player base, the harder modding becomes. It is not really in anyone's interest to see either fragment.
1. That is something the mod authors will be deciding, really - if their mod will work on different platforms.
3. Again, it's the modders choice in the end, isn't it? I'm sure Nexus will be filled to the brim just like the Steam workshop.
Can't believe I agree with what Tuco said but there you have it - you chose your platform knowing full well the pros and cons.