First, regarding your suggestion, a chance to bring Em and Nat together at the end of Chapter 5 exists! It super exists! It exists, has a bunch of variations and opportunities to steer things in various directions, and is 20,000 words long! In order to get the option, you have to have already raised Em and Nat’s relationship before the choice about who to spend time with at the end of Chapter 5. I’d very much be interested in having it tested, and if you still can’t reach it even when you’re specifically aiming for it, that’s something I need to know so that I can make adjustments.
As for your other questions:
The Sorceress wasn’t holding the Star shard, she was wearing it as a necklace, which is how it got in the wound when she was stabbed in the chest. She does say that, but it’s fairly quick, and I could probably make it clearer.
The scene with Vivi after the lightning strike is the main time to influence Em and Natalia’s relationship. There are a few other moments where you can do it as well, like in some one-on-one conversations with them, but that’s the biggest opportunity. That might be something I should tweak, especially if it’s too hard to get their relationship high enough to get their scene in Chapter 5.
Regarding immortality, I’d rather not go into too much detail as it could get spoilery (and I’m not totally decided on what will appear in-game and what might just be implied for the future), but granting it to other people is certainly a possibility.
Regarding Em, not absolutely decided (I don’t always work out the exact details of romance mechanics until I get there, because it can depend on how story and code things develop), but it will probably be the kind of thing that makes a romance harder and requires you to persuade Em to keep going than something that prevents a romance entirely. I tend to think it’s more interesting to be able to discuss points of disagreement with the love interests than to block them off completely, except for things that they’d find completely unforgivable under any circumstances.
You definitely don’t have to run in the election. There are a few different options for how you can spend your time as it approaches. And yes, you can certainly lose.
Current plan is ten chapters + epilogue.