No.
Okay, that may not answer much. Let's see if I can explain....
Soke, as a term, is, as you say, rather difficult to pin down. Like many such things, it is often easier to look at what it isn't, rather than what it is. And one thing it isn't is an English/Western term. Another thing it isn't is self-appointed. Thirdly, it isn't applied to systems without a sense of heritage and history. So a Westerner, making up their own system in the West, and appointing themselves as "soke"? Nup, all that shows is a desperate lack of understanding of the term. If it's a Western art, then the founder should have a Western title, really. As mentioned in our organisation, our Chief Instructor is just that, our Chief Instructor (oh, and Paul? Agreed that those are possibly better literal translations, but looking at the position itself, Kancho is probably a better term. Shihan and Hanshi would be the senior instructors in the organisation, although I can't see us using those titles, honestly... wonder what I'd be, though... hmm).
Back to your list of possibilities:
1-The title of Soke can be applied to a founder, but typically by later generations.
2-Menkyo Kaiden implies full transmission, Sokeship is more about ownership of the system. In a number of systems it is passed along hereditary lines (Takenouchi Ryu, Tenshinsho Den Katori Shinto Ryu, Sekiguchi Ryu), in others it isn't (Hyoho Niten Ichi Ryu, Morishige Ryu, Yagyu Shingan Ryu).
3-See Paul's answer here. Once more, though, Soke is ownership of the Ryu, so while not being the only source of the information, they are considered the source of authority for the Ryu.
4-Again, typically yep. Although there can be new traditions established who use the term Soke legitimately, such as Meifu Shinkage Ryu Shurikenjutsu, currently in it's second generation.