1) When you're viewing the stat +/- in-game, it's only taking the body piece into account. When you add in the head pieces, the heavy iron isn't better. The plain steel cuirass + steel Celata add up to much higher defense and stats, and the same number of materia slots (and if you're silly/rich and overmeld something so low-level, you get more total materia slots). Add in the Aetherial pieces and the difference is even greater. Heck, you can do better than the tin can with the DPS helmets (Steel/Aetherial Steel Sallet). Same is true of the heavy iron leg/boots vs. separate leg/boot pieces. Don't trust the tooltips in-game, add up the stats.

Le stats:
Heavy Iron Armor
Steel Cuirass
Aetherial Steel Cuirass
Steel Celata
Aetherial Steel Celata

Don't know about 2 & 3, I haven't done GLA or mages enough to research. I'd imagine healers would prefer tanks with high block rate, since it means smoother, more predictable damage, which is always easier to heal, even if the overall average is the same. But if it's enough of a stat jump, an upgrade is an upgrade.