(You can also play immersively as though you're unaware of them even when you play a Dunmer since the game's PC is described as having been caught crossing over the border into Skyrim before the game starts, so maybe you're just playing Nirn's least clued-in Dunmer from another region.) Those dialogue choices are just there if you want or need them, so if you want to act like you're playing a Dunmer who would already be aware of those areas or events, no problem, I think.
On the second question, as I'm sure you've guessed from the DLC's name, the main quest line and certain others heavily involve your PC being the Dragonborn and already knowing that fact, but, as far as I can remember, you're never forced during dialogue to act like you know nothing about Solstheim, the Red Mountain eruption, etc.
