Welcome to OCDO.
You might want to post your inquiry at the California sub-forum, where you can get responses from folks who are versed in the specifics of California law regarding shoulder holsters and visibility of speedloaders.
As for OC in an auto ith the seatbelts, you might want to look at changing the position of your holster. Try moving it closer to the 3 o'clock position (your nose is 12, where you sit is 6; your right arm is 3 and your left arm is 9 if you are not familiar with the clock designations) or even a bit more forward. This puts the revolver grip forward of where the seat belt/shoulder strap meet and click into the buckle end. If you like that for car carry bit not for walking around you might try using a paddle holster to make repositioning easier.