Is it common or uncommon for an Introvert to have So as dead last in the stacking?
"I absorb energy like a sponge everywhere I go. It allows me to see the world and my purpose in it." Zak Bagans, Ghost Adventures (INFJ)
About as common as it is uncommon, I think. I haven't seen a whole lot of correlation between placing of SO in the stacking and E/I.
5w6 or 9w1 sp/so/sx, I think
don't think we have statistics on this yet (though i would very much like @Jonnyboy to get his datamining skillz on the job), but so far - none that are clearly visible.
i wouldn't be surprised to see some strong correlations in the enneagram though, like 7's being more likely to be sx, 6's being more likely to be sp, or 3's being more likely to be so.