Few things to point out mate.
Firstly, I’ve contacted Flash via Discord to help with the attribute level requirements as they are not all present throughout the site (most available in skill simulator only), but even then it doesn’t appear all of the attributes have properly been synced with their skill level requirement. I can’t check since I don’t play KToS but if I recall you need lv2 for the attribute on Swell/Shrink. From memory at least, someone who plays KToS can correct me if I’m mistaken.
Secondly, the Shrink/Swell attribute is not a debuff that grants you additional MATK. This is a damage delivery attribute meaning when you cast it you will deal up to 60% of your MATK to the enemy. This translates, as damage dealing skill, into a wee bit over half of your auto attack damage. AKA #boxofshame levels of damage. If you have 1k MATK you will deal 600 MATK when casting the spell. This does not increase any damages done to the enemy for the duration of the size change.
Secondly, when you change the enemy’s size via Swell or Shrink you override their previous state. So you cannot have both debuffs on the enemy at the same time. As for the bonus rates… good question. I can’t recall receiving the double rates if I swell a shrunken enemy though. But I seem to recall receiving double rates if the size exceeded their base, i.e. you’d have to swell twice in that example. A M -> S via shrink, then Swell back to M wouldn’t give double, but if you Swell to L from base M then you’d get it.
Swell also doubles the enemy’s current AND Max HP though so do be careful. I killed quite a few afk+z players that way: Swell the enemy, once it’s half hp I’d Shrink + Swell again to fill it back to full. The only exception is a L size enemy, when affected by Swell, does not gain double HP but does still give double drops (2x of the same drop, not separate loot table rolls) and double exp. I am not sure if this is by design or a bug but it’s been brought up and brushed aside, and I’m not complaining either way lol
Lethargy for the attribute is pretty much required, especially if you’re going for a support role. While you may not have a lot in your tree that benefits from it you will be bringing a debuff that a lot of your party members may enjoy. So in your build example I’d take 1 point from EQ and add it to Lethargy. Given that you are going Pyro second you are correct to ignore Energy Bolt entirely.