Having a build that relies heavily on my follower + summons, are there any quests or areas that are restricted to your followers? Will I be forced to go it alone anywhere? Main culprits I would think would be sovngarde and the DLC.

Just Sovngarde, Apocrypha and Skuldafn. There's also missions where you're forced to take a follower besides the one you carry, but there's not too many of those.

About the only places you are forced to give up your followers are places or quests where the game gives you a follower. Even then, some of those let you keep the follower you have when you get the quest. Most of these are the quests you stumble upon, like outside of a tomb where someone asks you for help. Where the game is going to assign you a follower for a few quest stages, then you will have to dismiss your current follower for the new one.

And, yes, IIRC, Svongarde is a solo proposition. Dawngaurd has it's own follower for that MQ so sometimes you can use other followers sometimes you can't, Hearthfire lets you hire Housecarls (which can also be followers) and I honestly can't remember if Dragonborne is follower free or not (I never took one over there) but there are followers on the island that you can use.

No, you can bring followers to Solstheim, just when you go to apocrypha, you're svcked in alone. But they're still there when you come back.

Yes, you can take one with you. Both Jenassa and Ghorbash have gone with me on different characters.

Well - they will wait outside the dungeon in most cases. The Companions is the only questline I can think of where you actually need to dismiss a follower.

Twilight Sepulcher is follower free (and without quest follower), too.

Hint: If your character doesn't use Conjuration, make sure to do Sanguine's quest and get a very useful summoning staff first.

I can't remember, can a follower go with you Waking Nightmare? I think Ghorbash may have waited outside for that one, but I'm not sure.

And just a couple of quests with the Companions.

But seeing as you're a magic user, you're probably not cut out to hold some steel.

No, he/she will wait outside. But you have Erandur with you through the quests, and he's quite efficient for the enemies you'll encounter.

Followers will never come inside Snow Veil Sanctum, Irkngthand, or the Twilight Sepulcher, even after related quests have ended. Just discovered that on my current playthrough

Come to think of it, I think this might be the case with Nightcaller Temple as well (with the exception of Erandur, obviously, since he lives there). So this is probably true for all "non-follower" dungeons.

Which doesn't matter. Can't die in Apocrypha anyways. You seems to wake up on Solstheim after "You can no longer fight" message. (I've once tried to commit suicide in the world by swimming in acid. Didn't work out as hoped).