Hi.
1) I am same as you, I like to work alone. Working with another is irritating, to say the least. I always think I can move faster when I don't have to communicate with anybody else.
But. Doing business is less about doing stuff well and more about avoiding really stupid shit. Having other people to work with and especially a cofounder is one of the best ways to get feedback for really stupid stuff you could do.
It is all about reducing the time and cost between when you become wrong and when you stop being wrong. For some reason it is very hard to do it on your own.
2. Don't set milestones. Understand the problem you are trying to solve for your customers. Do your best solution of the problem, the best way you can. The customers are just validation of your idea -- if they come you might be onto something and if they are not coming you are doing something wrong.
3. No. It is up to you. If you are asking this question you haven't thought this solopreneurship thing yet. The whole point of doing it solo is to be able to make those decisions yourself.
4. The best way is to make a good product/service. The customers will come. Don't focus on the customers, focus on the best service you can. The more customers you have the harder it will be to focus on the service, so cherish the ability to move fast while you have no/few customers yet.
5. You want to slowly build it up. Financing is a hack that should only be used by people with experience -- not you, at least not yet.
Your goal for your first business should be to avoid getting killed financially (don't get into debt!). As long as you do that, you have ability to iterate. Most of successful people I know have started multiple businesses before they finally succeeded. That assumes ability to start another business after the first one fails. Bankruptcy will very likely put an end to your career as an enterpreneur.
Also, slowly building it up gives you time necessary to learn, to transform yourself. And build a product. It takes time to think all those things and integrate it into your experience and trying to hack it will just leave you woefully unprepared.
Also, when you get financing it suddenly puts a timer on your business. It puts a pressure that makes it difficult to focus and make your own decisions. Everything from now on is a compromise. And, at least in my mind, this conflicts with trying to build the best product you can.
Also, when you get financing, the cost of that financing will depend on how risky the investment is (I am kind of expert when it comes to risk). You don't give loans for nothing, you want to earn some money. And the riskier the investment, the more you want to earn. And as a first business with an unexperienced founder, running solo, you are as risky as it can be. If you ever get somebody to invest money in your business it will necessarily come at an exorbitant price and you are also very possibly get abused by somebody with much more experience than you.