Us getting Judaism in additional Cities like Corn and Cap can mean not only 40 free Hammers but also several free Hammers in Buildings (hard to quantify, but let's say an average of 30 Hammers per City in between the time of Mansa sending Judaism to us and us manually spreading Judaism to our Cities later).
Further, it gives us flexibility for spreading Judaism... if we spread it manually all from Pig City, it will take longer than if we can spread it from 2 or 3 different "source" Cities.
It's hard to say what Genghis' Workers will do, but they will probably finish their current tasks and then consider moving to the Copper... which may mean next turn or it may mean a couple of turns from now.
We have 70% of Mansa's Power according to the BUFFY info on the Scoreboard.
If 1 Warrior = 2, 1 Archer = 3, and 1 Skirm = 4, then we have:
4 * 2 + 3 * 3 = 8 + 9 = 17
17 / 0.7 = 24.25
24 / 4 = 6
So, Mansa has 6 Skirms... 2 per City... and thus may not even have a free unit to send toward Joao at all... at most 1 free unit, since 2 must stay in his capital, 1 must stay in his second City, and 2 must stay in his third City until he's owned it for 10 turns' time.
If he goes to war with Joao, Joao may Pillage him.
If he goes to war with Genghis, he's far more likely to build units that will get trapped... thus, he'll be more likely to build up an army in relative peace (for use against Joao later), while still being able to send Jewish Missionaries our way.
I can upload the odd small screenshot to CFC, if I keep deleting older uploaded saved games, so here's one of Genghis' area:
Unfortunately, my normal image server still seems to be down.