There are two misconceptions in your examples and there is another misconception in the reply to your question by anna v.
The first problem concerns what happens when a person chooses whether to turn left or right. If the person in question has a reason to go to a specific place and he has to turn left to do so, then he will turn left with extremely high probability, quantum fluctuations in his brain notwithstanding. Your brain did not evolve to be extremely responsive to random noise since that would get in the way of it helping you spread your genes.
The second problem is with the way you say the particle "assumes all points in its probability space simultaneously". This is not very clear. In the MWI the structure of the multiverse is determined by the flow of information. A universe is a structure within the multiverse in which information flows freely. So there may be a version of you that is sitting one millimetre to the left of where you are currently sitting but you can't interact with him owing to decoherence and so he is in a different universe. But at some point for two versions of you that are close enough, they can still interfere and so you and that "other version" are not in separate universes. For any particular purpose you have in mind there is some finite (but possibly large) number of versions of any particular system you might interact with.
The third problem is that you seem to regard the number of universes as relevant to judging whether the theory is true. It is not relevant. If we're just going to bar explanations with large numbers of entities then presumably we should dump the atomic theory and the theory that other stars exist since there are large numbers of both. And in any case, what is the standard for judging that a number is large? Is 100 a large number, or 1000, or 1 billion?
Finally, many people seem to think that the MWI is some optional add on to quantum mechanics. It is not. It is just what you get when you apply the theory to macroscopic objects. Now, anna v claims that the MWI makes no testable predictions but this is false. This can be seen by contrasting it to, say, the Copenhagen interpretion. The MWI claims that some quantum mechanical equation of motion will apply to any system you study including measuring instruments and your own brain. By contrast, the CI claims that somewhere, somehow, in some unspecified way, the equations cease to apply. The MWI makes precise predictions, the CI doesn't. The MWI has been tested, and will continue to be tested, in experiments on decoherence. And the idea that quantum mechanics applies to macroscopic objects solves problems that the CI does not, such as how correlations are established in EPR type experiments. See David Deutsch, Patrick Hayden, 'Information Flow in Entangled Quantum Systems', Proc. R. Soc. Lond. A 456(1999):1759-1774. available at http://arxiv.org/abs/quant-ph/9906007. And also David Deutsch, 'Vindication of quantum locality', Proc. R. Soc. A 468(2012), 531-544. available at http://arxiv.org/abs/1109.6223.
I don't agree with everything David Wallace says but he does explain some issues correctly
See also many of the papers and books listed here