Ok more review, sorry this is so dispersed.

[PATCH 14/42] mkfs: factor printing of mkfs config

Oh good.  :)

[PATCH 15/42] mkfs: factor in memory superblock setup

Heh, I read that as factor in vs factor out ;)

This seems to change logic slightly, is it intentional?

from:

-	if (sb_feat.inode_align) {
-		int	cluster_size = XFS_INODE_BIG_CLUSTER_SIZE;
-		if (sb_feat.crcs_enabled)
-			cluster_size *= isize / XFS_DINODE_MIN_SIZE;
-		sbp->sb_inoalignmt = cluster_size >> blocklog;
-		sb_feat.inode_align = sbp->sb_inoalignmt != 0;
-	}

to

+	if (cfg->sb_feat.inode_align) {
+		int	cluster_size = XFS_INODE_BIG_CLUSTER_SIZE;
+		if (cfg->sb_feat.crcs_enabled)
+			cluster_size *= cfg->inodesize / XFS_DINODE_MIN_SIZE;
+		sbp->sb_inoalignmt = cluster_size >> cfg->blocklog;
+	}

it loses the test for inalignmt being 0 and turning off inode_align...
if you deemed that impossible, a commit log comment would be helpful.
If not, maybe it's a bug.  :)  The test & set goes back to the mists
of time, so I'm not sure of its purpose.