As long as Cloud is equipped, if an ability of Cloud or an Equipment attached to it triggers, that ability triggers an additional time
I did debate on just filling the rest of the manabase with basic lands, but ultimately, I felt that restricting the nonland spells to (mostly) Golgari guild cards was already keeping the overall power level down, so adding the best lands I could to the deck was fine
You can also use it in your inkwell, but unless the situation is dire drawing cards is almost always going to be the better choice
Unlike the Thick Grey stamp which may have formed by stacking printing sheets before they were properly dried, I think Thin Grey stamps are indeed due to being low on black ink
Skilled, experienced deck-builders have some developed statistical intuition and, of course, do not need to calculate such statistics all the time
There are stock alert pings, people sharing successful finds in the wild, and links to lotteries (like the Amazon one) run by other independent shops