Evaporation would probably still be massively overpowered at 3. It can mimic several (good) level 6 spells.
Realistically, at 3 mana, Evaporate is going to be used as a slightly more costly version of Mephetic Cloud or a really cheap version of Poisonous Cloud. While theoretically you could produce Freezing, Mutagenic and Flame clouds, in practice you weren't because you didn't have an unlimited supply of those potions. I can see the argument that 3 mana is too cheap for Poisonous Cloud, but you have to keep in mind that you're expending limited resources (poison potions) to pull that trick off. It's not like you could spam it freely at that price.
Dig is a situationally fun spell/ability. If you have some way to see through walls (Ashenzari, Deep Dwarf, a lucky Demonspawn mutation, or, more recently, Formicids) then being able to path through said walls is highly useful. If you don't have that ability, you're basically only going to use it for digging boltholes, which is less exciting, but still a valid tactic in my eyes.
Summon Scorpions was important for Venom Mages wanting transition into Summoning instead of Conjuration. It was also the go-to tactic for Summoners facing the Spider Nest, an early Ant Vault or Killer Bee swarms, given that it was the only summon with poison you're guaranteed to have access to. It also had the advantage of letting you put out a buttload of chaff at once, since it could do up to 8 scorpions.
Demonic Horde was the high level version of Summon Butterflies, Putting 8 imps in between you and the Executioner in one turn was much more useful than laying down 8 butterflies, since they had actual resistances and could occasionally take more than one hit to kill.
Shielding and Ugly Things, were admittedly, not very useful. I could see cutting them, if there really wasn't a way to make them useful.
It takes ages to get two stars with either TSO or Zin, I'm not sure if you'd even get that before temple.
You'd start with two stars of piety, just like Healers do.
This is weird because you haven't actually said what the problem with their current approach is, other than the fact that their approach involved changing a few spells.
The issue with summoning is that the dev team hates it when players use tactics to minimize their exposure to danger. They hate pillar dancing, they hate hiding in corridors, they hate attacking stuff that can't see you and they hate anything that can be used as a barrier between the monsters and the character. With summoning, that's all there
is to the school. The whole point of summoning monsters is so that you can hide behind them and not get attacked, or use them to break chases, or get them to eat hits for you. Players want to use the spells for things the devs don't want the players to be able to do.
The current approach to "fixing" this appears to be: A] sharply limit the number of summons a player can have, and B] replace all of the good chaff summoning spells with silly gimmick spells like Guardian Golem and Lightning Spire. That doesn't really fix the problem at all, it just means that players need more spell types and to spend more mana to get the desired amount of chaff going. A better solution would be to make the chaff itself dangerous (disloyal summons), to introduce a choice between being safe and being able to kill things (summons that ignore you if you run out of mana) or to give players a reason to not treat summons as expendable (permanent costs for permanent summons).