At least for me, as a western left-to-right, top-to-down reader, your idea of having the help come before the input would be weird, as I expect the focus to advance in the same fashion I read, that is, left-to-right, top-to-down. Breaking that order for jumping between required versus optional fields is also disorienting - if the optional fields are less important, move them downwards on the page and don't play funky games with the tab order.

The tabindex should only be used if funky CSS is used to rearrange the page layout.