Ah, I'm glad to hear the Schrodinger's Butterfly issue has been solved. Trying to catch them when they're popping in and out of existence at random is, while a sporting experience, not all that pleasant. I have noticed another, possibly related issue. While you've stated these butterflies are supposed to work the same as Vanilla ones, I've noticed that, unlike Vanilla critters, and despite having the Consumable tag, you cannot actually use them to set them free. Holding one in hand and attempting to left click does nothing.
Speaking of Butterflies... This actually has nothing to do with Butterflies, but they're tangentially related to the Summoner class so my point stands.
I've been playing through Thorium as a Summoner, and I have a few suggestions and thoughts.
(Note: I have previously mentioned some gripes with how Thorium's summoning weapons work differently in comparison to Vanilla summoning weapons. I will not restate those here as they've already been noted.)
A few thoughts I've had:
Firstly, I feel the Strange Skull (and likely the Phantom Wand due to functioning almost identically) are a tad too difficult to use. In any confined space, they tend to appear inside walls and as such, cannot shoot, immensely limiting their effectiveness in anything but the widest open of wide open arenas. Sometimes they will even spawn under the arena floor if in a wide open arena. I feel this could possibly be rectified by giving their projectiles similar wall-passing ability to the Starfury's stars, wherein they will pass through a wall if they spawn inside one or there's one right in front of the skull, but after entering a wider section of open air, they collide with any other blocks fine. I feel this would maintain the balance while allowing the weapon to actually be usable in 60% more cases.
Secondly, I feel the Void Lance is a bit underpowered, honestly, considering you need to beat Coznix to get it. Mainly its range is absolutely horrendous. The gate only seems to attack things that are within arm's reach of it, and that makes effectively using the weapon a bit of a pain considering there can only be one gate at a time.
Thirdly, I'm honestly surprised there's no Gnome summoning item. The Gnomes found in Hallow are essentially the perfect size for summons, much like the Vanilla Pygmies and Pirates. I feel this is a missed opportunity. One should always have the ability to harness the power of Gnomes. I even have an idea for how it could work. It could summon a Gnome Horde like the enemy, which would "shoot" Gnomes out of it at enemies, that would latch on and deal damage, or if they miss, run back to the main group.
Fourthly, another thing I'm surprised at the lack of, is an upgrade to the Phylactery. While (to my knowledge) the earliest Death-Preventing accessory there is, it's also a vital Summoner item due to its increase in stats. However, its Death-Prevention is... well, terrible. All others have a 2 minute cooldown and restore at least 100 life. The Phylactery, however, restores a measly 20 life, barely enough to survive another single zombie strike in Expert mode, and has a whopping 5 minute cooldown. I think this is perfectly fair. However, the lack of a later upgrade annoys me, especially when there is the perfect opportunity to add one: The Lich, who has a Phylactery of its own. I suggest a Hardmode upgrade to the Phylactery with the following attributes:
Phylactery of 100 Souls
Accessory
'An artifact fit for a true Lich...'
Increases your max number of minions by 1 (No change)
20% increased minion damage (from 10 to 20)
20% increased minion knockback (from 10 to 15)
While worn, taking fatal damage will instead return you to 100 life (from 20 to 100)
This effect will recharge every three minutes (from 5 minutes to 3 minutes)
Crafted with:
1 Phylactery, 10 Cursed Cloth, 25 Souls of Night, 25 Souls of Light, 20 Souls of Flight, 10 Souls of Sight, 10 Souls of Might, 10 Souls of Fright, Summoner Emblem
It would still be weaker than other such items, having a whole minute extra cooldown, but with a reasonable amount of health returned, and slight buffs to the pre-hardmode Phylactery. In addition to the cost to craft it, and when it is gainable (post-Lich) I think it would be more than reasonable. In fact I think my crafting recipe may be too harsh, but i wanted to have a semi-even split of 100 souls to fit thematically.
EDIT 12PM: I forgot to mention this last night, however I think it should also add at least 3-5 Defense, maybe at the cost of 5% Minion Damage, or perhaps give a 5 Defense boost while Lich's Withdraw (cooldown) is active.
This is because Summoners, contrary to what Re-Logic seems to think, have horrible survivability. Despite having less defense on average than Ranger and Mage classes, they often need to be MUCH closer to bosses to deal damage effectively, as many minions can't even target the edges of the screen, much less off-screen, even at 1280x720 resolution.
Due to their haphazard targeting and pathfinding, battles are often drawn out significantly longer (which is also why I suggested such a large damage boost; I find that from Coznix to the Mech Bosses, where I am currently in my playthrough, the damage output is somewhat lacking even with my highest DPS non-piercing ranged minions and manipulating bosses into their slow attacks. Granted, I don't have a Summoner Emblem, but I feel a 5-10% boost is not that bad).
EDIT 11PM: After progressing to Post-Golem, I find that wow, the Black Cane is incredibly good. An amazing step up from previous weapons. Fast, strong, that's nice. I also notice that there are a LOT of damage-boosting accessories for Summoner. Necromantic Scroll, Hercules Beetle, Papyrus Scarab, Yuma's Pendant, Minotaur Skull, Summoner Emblem, Phylactery. And yet, despite this, I realize that due to Summoning weapons having low base damage, the increase is... Really not that astounding, unless you have every single one of these equipped.
As such, I stand by the damage increase despite many other items having similar damage increases, since Summoning Damage doesn't get as big a benefit from damage increases. I also notice late-game survivability is even worse than I thought. After upgrading from Life Bloom Armor to Tiki Armor... I promptly reverted since the Life Bloom effect is far more useful than a measly few defense, a negligable damage boost, and one minion slot. Upon upgrading again to Spooky Armor, I found a nice damage increase, at the cost of a MASSIVE 10+ defense and the Life Bloom special effect... So I suck with the Life Bloom again.
My point is, I guess, that Summoner feels lacking in upgrades. New accessories and new armors don't feel that "new" and in most cases feel like more of a sidegrade, aside from a few tier bumps. It's quite unfortunate how the class is designed, although most of that blame goes to the atrocious Vanilla Summoner "tiers". They're weak as heck until Plantera, where suddenly they become unstoppable powerhouses, and that's only exaberated by Thorium (although in a good way, progression feels more natural and smoothed with Thorium.)
In addition, it would be an immensely useful Quality of Life change to allow Death-Prevention cooldowns to be reset by the Nurse, or perhaps by some other craftable item that requires holding it down for 10 seconds while standing perfectly still or something, to prevent people spamming the Nurse to be immortal. Just so after a big battle where you used it, you don't have to sit around and wait for it to recover if you want your Death-Prevention back.
...Apologies for the massive wall of text. It's 3:20 AM and I tend to ramble when tired. Edit: Apologies for the multiple wall-of-text edits too. I can get rather passionate about stuff I enjoy.