Unfortunately the ban on mineral spirits, white spirits, etc., in Southern California limits your options. That's over-regulation that will tempt some folks to use much more hazardous materials.

Keep in mind that "paint thinner" is such a generic and vague term it can mean anything from alcohol to petroleum to water based solvents. Even "mineral spirits" is an imprecise term and the actual ingredients varied quite a bit, including in flash point.

If I was forced to do without mineral spirits I'd use citrus or simple green type cleaners, and 91% isopropyl alcohol when needed for degreasing and removing any residue from other water based cleaners (which includes the citrus and simple green type stuff). For my spinner brush type chain cleaning tool I use a homebrew of water based degreaser (Dawn or Stanley) mixed with 91% isopropyl alcohol. If using 91% isopropyl for anything more than dampening a cotton swab or cotton ball sized pad or cloth, it's best to take it outdoors away from ignition sources. Flaming alcohol can be scary stuff because it's almost impossible to see in bright light.

MEK is so hazardous it should never be mentioned in the context of an introductory level discussion other than to warn folks to avoid it completely. If it's necessary to explain why, it's the wrong material for the job. Even if you could use it, MEK is probably the wrong stuff for most bike maintenance. It can damage many plastics and is used in welding some plastics together.

For complete information it's preferable to refer to NIOSH information -- readily available online -- in preference to a Material Safety Data Sheet. MSDSs are prepared by manufacturers and sellers, and are prone to errors and deliberate omissions. A manufacturer can omit information they believe to be proprietary or trade secrets, legally or not, correctly or not, and the end user might never know. NIOSH info is more complete, unbiased and non-partisan.