Your fixes for the Acid and Fire sound great to me.
I get what you mean about an Ice zone have mechanical overlap with the acid pool, it would require a little more programming but you could add a "shatter" mechanic to the Ice effect so after a set amount of time a frozen enemy would break into shards that traveled for a set range that did damage to other enemies on contact.
For balance you could make it start with a single shard and every 2-3 levels in Ice would add an additional shard. Also as long as your projectiles and ice effects are on separate physics layers you can just stop them from interacting via Unity project preferences (this way other enemies and the player are still blocked but projectiles pass through).
I'm not sure the Ranger, Knight, Wizard summon idea is super cohesive with the rest of the game/mechanics. It feels like it's being shoehorned without a meaningful reason. (Granted you could have a story element in mind that I know nothing about, this is just my opinion based off the blurb about "Helpers with summoning sickness").
I like the idea of adding towers but I think something like a totem might be more fitting, you could allow the player to choose a element (or maybe a spirit animal) and depending on their choice you could offer totem upgrades like your current system or a even a skill tree.
For example if you used a element system:
Nature (Acid) has the Forest Totem that has skills to confuse the enemy, heal the player, do an AOE slow, etc..
Ice would get a Magic Totem that could shoot piercing Ice projectiles, do weak AOE damage (like a blizzard effect), after a charge it could an add Ice armor to player that could absorb a hit, etc..
Fire would Grant a Lava Totem that could provide an area where the player could move faster, it could randomly fire lava projectiles like a tiny volcano, boost the
players attack speed, etc..
I'll try to hop on at some point this weekend and check out up the update!