Having been in these straits before, I'm going to give advice that's a little different.
The key to starting a business is less about aptitude and more about ability to have a market. The only difference between you and someone who is doing what you want to do already is they have established relationships that enable them to get work consistently enough not to be "living on the bare minimum" for a while.
I think the nature of your questions means that you don't necessarily have all of these things into place yet and that you're hoping your moxie/hustle will get you through. While it might for a while, eventually you'll come to a realization if it's not working, that you need to do something else quickly for money and you won't have the capital to make that possible as much as you might right now.
My suggestion? Find work with a more established person in the field you want to be in. You need experience to answer the questions you have, even if you have some already. More importantly, you need to get the handles down of workflow and see how other people run their businesses because the market opportunities that exist while other people make mistakes is how you develop a product or service that you believe others will pay for.
As for whether to inform employers that essentially, "I'm going to take this job for now, but my dream is to do my own thing, so I might leave in an unspecified time when that happens," it'll just make someone not want to offer you the job. Maybe someone will appreciate your talent and see you for who you are and mentor you. But these scenarios are few and far between and even if they happen, without a lot of hand-holding and guidance, it just leads you to think you're closer to where you want to be than you are.
The key is getting your reps down and getting a sense of why you'd be able to establish yourself in this industry better than someone with more money, contacts and so forth.
Right now, you need money and that way, you can continue hustling your dream in the hours when you're not at work and on weekends and so forth. Not to do that forever, mind you, but for where it sounds like you are, it might make the most sense and get you a lot further along faster than hanging a shingle at this present time.
