Astral 4 - Magic Resistance +1 (Max 18)
Astral 6 - Magic Resistance +2
Astral 8 - Magic Resistance +3
Astral 9 - Twist Fate (protection from first hit)
Astral 10 - Magic Resistance +4

That is the Astral Bless effect chart.

Luck does allow you to get heroes of a greater chance. Though I would be taking luck for just that point. Order reduces the likelyhood of a random event by 10% per scale.

As for your scales and pretender; it's whatever you'd like to play. Arco are very strong, you have to decide how you like to play before you know what you want to optimize.