First Part
A big piece of advice is to write what you want to write. If you just write what others tell you to you're much more likely to lose motivation which will end with nothing being written. Further, almost any idea can work, and the two you mention both sound like they could be great, but since you are entertaining the idea of the latter, I'd encourage you to go with it.
Another big piece of advice is to keep the scope of your story in mind, and consider planning things out. You want to find what works for you, but the thing with branching stories is that they can get out of hand extremely quickly, which can, again, kill motivation. Having some sort of plan/outline can thus help you moving towards a specific goal to prevent you from having the scope get out of hand.
Alternatively, consider a smaller scope story to get used to the process. Starting with something smallish is often suggested because it helps you get a feel for how CYOAs can spiral out of control, and thus stop you from languishing in some epic.
Second Part
This is up to you, both can work, but you pretty much want to figure out what role you want realism to play in your story. Think about some specifics for ideas regarding what can happen and what sorta branches you want to have to help you find the preferred choice. As you say that fantasy elements will make the story more enjoyable & captivating, I'd say go for it, since it sounds like that is what you prefer. Again, write what you want to write.
Third Part
Again, up to you, and write what you want to write. A great thing about writing is that you can go back and change things if they don't work out, so perhaps try writing the opening with a narrator and without and see which one you prefer. Same thing regarding technology (this also ties into what role realism will play).
Conclusion
You aren't going to get experience if you don't write, so ultimately, don't forget to actually start writing at some stage. Some people like to plan a lot, but it is about finding what works for you, and regardless, in the end, you'll need to actually write at some stage if you want it to get written (unless you somehow have people to write for you, lol).
I'd say, try writing in a number of different ways and styles, it is the main way to improve and figure out what works best for you. In this case, maybe write some short stories trying out the different ideas you have? Have 1000 words as the upper limit to ensure you give them all a shot, but obviously if you find one clicks with you, feel free to just keep writing.
Looking forward to seeing how this idea develops.
TL;DR
Just write what you want to write! Try different things and change them if you aren't happy. Motivation is vital, so writing what you want to write is immensely beneficial. Figure out what works for you through experimentation, there is no real wrong way to write a story (tho do put in some effort, after all, if you just roll your hands over the keyboard, chances are slim that you'll write something good).