The f-statistic is just for visualization purposes to look at differences in relative abundances. Differences in relative abundances do not necessary reflect differences in absolute abundances. So this is not a statistic that I would recommend reporting.
Here are the posts where the ANCOM W-statistic is discussed in more detail
Not necessarily - just because you have 10 phyla that are significantly different, it doesn't mean that the other 22 phyla aren't significant. It just means that you couldn't rule out the possibly that they aren't significant. Maybe worthwhile reading up more about statistical hypothesis testing in addition to the supplemental section in the ANCOM paper