I really don't see any other option but to run. If it was just the black dragonspawn (I'm presuming raiders, because 26 of anything else would have stomped your party) I'd give you even-ish odds but then there's that spellcaster. Whoever he is, he's the one who killed the adult copper, 'cause a bunch of non-flying CR4 dragonspawn sure didn't, which puts him around CR10-11, which would be darn tough for your party without the dragonspawn. On the plus side, if he just killed an adult copper, he's probably fairly drained (if he's not you're doomed anyway). So yeah, time to run.

I'd say your best bet is to have the druid go central and try to hold the dragonspawn while the fighter/cleric/anyone else really hurt make a break for it. He can probably hold for a round or two because he's got the wolfs, the buffs, he hasn't really been hit yet, and he's a freakin' druid. If the chain fighter isn't too hurt I'd have him help (with dragonspawn at 60hp a pop I'd focus more on tripping/slowing than damage) and have the bard use Tasha's Hideous Laughter on any dragonspawn that just has to be removed for the fighter/etc to get away. Once the wounded have gotten a headstart, have everyone run for it (remember that if you really need to get out of dodge, the druid can shapechange into a horse as a standard action, chainfighter/bard can jump on with readied actions, then the druid/horse can move out of there (I'm pretty sure that works)). Hopefully the summoned animals left behind can buy you another round. I doubt the spellcaster will give much chase if he's already drained from killing the copper. The only real danger is if he casts a Web spell or something.

There's a real risk because most of your team moves at 30ft and the dragonspawn go at 40ft (and know it). If they give chase, you best bet is to find a treeline or hill, set a quick ambush, drop one of them with focused long range fire, then hold them off with summoned animals while you all run again. If all else fails, the cleric's sanctuary might be worth a shot.

I will say, there's a compelling argument in favor of circling back and attacking again. If your Barb/Fighter is cleaving through 4 dragonspawn, there's a decent chance he could one shot an arcane caster. And if this is an arcane caster who has shot most of daily spells on a copper, this could be your best chance. On the other hand, if it's a cleric, you're better off waiting for another day. Anyway to tell what kind of caster you're dealing with would be really valuable. Just be much sneakier than your dragon shaman was.

As a caution, there's a chance that there is no spellcaster. I think you're right about it, it's certainly the most logical explanation, but DMs come up with odd things all the time and the fact that he's only been blocking your magic (instead of offensive magic or focusing on the druid) and has only blocked haste spells (even if you haven't cast anything else) strikes me as odd.

On the metagame level, I think your DM is trying to get you to run. I imagine he wanted you guys to observe the cult instead of charging in headfirst, especially since you were already on another mission. His first instinct was to just throw numbers at you (I can't imagine he wanted to run a combat with 20+ creatures) and when that didn't work, he used psychology. A good portion of your group seems pretty optimized and when there was an indication of a spellcaster, you guys freaked out (reread your post, these things are close to killing your Barb/Fighter and the most critical thing to you is that someone is dispelling your hastes). If so, props to him because he knew just which of your psychological buttons to press. Then again, I don't really know your group but it seems reasonable to me.