There is only limited number of tags you can place on one map. This number isn't the same for all tags.
: Error: Array Full in C:\myth2\MYTH 2\definitions.c, line
: #301: there are no more free lightning systems
: definitions for '13 magic weapon 2ndary light'.
This means you already have maximum available number of lightning tags on your mesh. The limit is 4, I think. Both "The Summoner" and "Twice Born" have 4 lightning tags on it (2 for fetches, 2 for Alric and 2 for lightning towers), so if you add Balor or any other unit that uses different lightning tag on one of those levels it won't start.
: Error: Array Full in C:\myth2\MYTH 2\definitions.c, line
: #582: projectile definition postprocess failed on '13
: magic weapon sec projectile'.
Probably projectile tag is either corrupted or not properly linked to some other needed tags (collection, object, lightning etc.). Same thing appeared to me when I tried to assign The Ibis Crown to other monster than Jman-if you open its artifact tag in Fear, you'll see that there is no collection-16 crown doesn't appear on the list.