I am self employed as a Freelance Illustrator, but I did ask my most recent and consistent (US based employer) to create a letter explaining that I have been and will be working for him and could take a break. I have a book I illustrated coming out in about a month.
Am I allowed to do that if I'm not working for a non-UK based company?
No, it is illegal for you to carry out any work-related activities while in the UK as a visitor. Even if it's remotely for a US company and even if it's unpaid.
So, you cannot do anything to do with illustration at all for the entire duration of your trip to the UK (unless it's just as a hobby and you will not be making any money from it in the future - i.e. you could do it for your own entertainment, but you can't illustrate anything for a client or that will appear in a book), otherwise you will be breaking the rules of your visa.
As for living arrangements, I've lived with a friend of mine in Virginia for about 6 years with no agreement. I just paid her rent and paid for a few bills. I DO have medical bills I am currently paying that go to my address, can I use those as proof of return? Because legal action most certainly would be taken if I did not.
Well, you have to prove that you have commitments that mean you need to be physically present in the US after your trip and that you cannot stay in the UK because of those commitments.
For example, things like: attending an important appointment or interview, returning to work, attending classes for school, going to an important meeting or perhaps a family wedding/graduation, or maybe you have a child or an elderly relative that you care for who is relying on you to return etc.
If you can prove that you must be physically in the US to pay your medical bills (and not just pay them from abroad), then that may help.
For your home, can you get a letter from the friend who you live with stating that you are living there, that you contribute to rent and bills and that you will continue to pay rent while you are in the UK and/or will return to live there after your trip? What about evidence that you have paid rent/bills to her?
For your job, ideally you would want to prove that you will not carry out any work at all while in the UK, but that you have work already lined up for when you return to the US.
How long will you be staying in the UK for?
You said that you are not getting married until October 18th, but if you are applying for the visa now, I'm guessing you will be trying to stay for the majority of the 6 months, since October is 6 months away and the visa cannot start any later than July (you can apply online a maximum of 3 months before you wish to travel to the UK and the travel date you put on the application (which must be within those 3 months) will be the date that the visa is made valid from).
Essentially, the longer you try to stay in the UK, the more evidence of ties to the US you will need to show. For just a short trip, maybe up to 4-6 weeks, your application won't be scrutinised so much, but for trips of a few months, they will need to make sure you are a genuine tourist and not trying to use the visitor visa to 'live' in the UK without a proper visa.
So they will be asking questions like: How is he able to take so much time off work to visit for this long (most people only take short vacations)? How will he be able to support himself for so many months without working or earning any money while he is in the UK (most people can't afford to take several months off to visit the UK)? What about his home in the US, how is he able to leave it unattended for so long? What evidence is there that he won't try to stay in the UK illegally because he has no reason to go back to the US?
What are your plans for after the wedding? Where will you live together - will she be moving to the US or will you apply for a spousal visa for the UK, or will you move to a third country? Having evidence of what you plan to do afterwards and showing that you know the visa rules for what you plan to do can help too (i.e. that you know how to apply for a US spousal visa, or evidence that you know you must return to the US to apply for a UK spousal visa, or evidence of plans you are making to move somewhere else).