Harmonize is leagues better than Concentrate despite being the same card because green can happily spend four mana at sorcery speed with no expectation of tempo returns
If the target of Lazav, Wearer of Faces's first ability is illegal as the ability tries to resolve, it won't resolve and none of its effects will happen
Hopefully Wizards can address them in the sets to come.
In a deck with lots of other mill effects, you can double the amount of cards your opponent mills, giving you more options for spells to cast
Stalwart Unity is more of a personal preference than anything else, because I like group-hug-type decks that look to pillow fort, draw cards, and then hopefully win the game eventually
Another surprising payoff for this strategy is Collector Ouphe