There are so many questions in there that are going to be very situationally dependent. If you have not already written a few novels, I would say absolutely do not write your grand epic yet. You won’t have the skill to do it, and it will disappoint you. I’ve run across a lot of new writers who this has happened to. They want to do their own Wheel of Time, but they don’t yet have the skill to achieve it. I tried this myself and learned this the hard way.
That’s not to say that it’s impossible to do, but I strongly recommend to most writers to try a few other books first. Write standalones or something to really get your head around the idea of characterization and plotting and narrative arcs before you say, okay, I’m going to tell a story across ten whole books.
If you are confident of your skill, and find that you are just incapable of writing anything else? Writing is the most important thing. If something makes you not write, then it’s usually going to be bad advice no matter who it comes from. So then I suggest just writing and loving what you’re writing. If you can somehow style your book as a standalone with sequel potential, then that’s probably a better way to go.
This is not just for publishers and agents. New readers have a built-in skepticism toward a new author who is trying something that massive. I’ve found that a lot of readers like to try the standalone to find out what kind of writer you are, before they then read your big series. Having a couple of standalones has been very useful for me for that reason.
At the end of the day, just write what you love. Yes, editors and agents say they want shorter books. This is because historically it has been proven to them that authors trying to write books that are too long for them bite off more than they can chew and the book spirals out of control. But the draft of Elantris that was the first thing I sold was 250,000 words. That’s a full 100,000 words longer than everyone was telling me agents won’t even look at. So by empirical proof: They will look at a longer book if it works for them. So write what you love—if you can get into your head that you’re going to do this professionally, and that you have years to learn how to do this, then that’s going to help you. Taking the time to practice with shorter works will help you get ready to write your epic. But if you just can’t do that, then go for it.