How plausible/possible is it for gender to be included in the 15th Amendment, or for it to be phrased in a way that makes it acceptable to let women vote. If the latter, how long for a Supreme Court case to appear and make it standard across the country.
It's definitely not at all impossible, TBH. After all, women's suffrage had been an idea since the late 1840's(Seneca Falls convention in Upstate N.Y.) and nobody had raised too much of a stink about it yet, at least compared to blacks gaining official voting rights in all states(and not just a few states like Massachusetts, or New York).
I do think, however, that it may sadly be likely that women may end up being subject to restrictions based on state; you might not see much of anything in more socially liberal states like Vermont or Maine, but more conservative states like Georgia, Mississippi, and Alabama might very well make female voting near impossible(just as with blacks IOTL), and even some states more in the middle could fall prey to such to a notable degree as well. Of course, nothing is necessarily set in stone, though; just my two cents.