How exactly does holy symbol work now (Post bb)

That's correct, I can confirm that the best exp you'll get is when you have 5 afking leechers while you alone solo for kills. However, this is just pure numbers for when you kill alone. It is still possible to have 4 afkers while 2 people split the map but, the bonus exp that each person receives when a monster dies that they didn't kill will be smaller than when just a few people.

I see, so the more people "that can leech" the better?

So best = 5 afk leechers (who can leecH including the hs'er mule who can leech?)

What if I have a low lvl priest who can't leech? What will happen then?

What do you mean at the last part, if 2 people split map, then they will all gain less?