Rescues **may** be able to identify some problems -- particularly a problem that is visible at that point in time. But please don't lose sight of the fact that rescues are *volunteers*. Generally people who like dogs and may love the breed -- but they aren't trained health practitioners.
"Well don't they go to the vet?" Yes, BUT ... vets may work at a discounted rate for a shelter, but testing for some sort of future eye problem would be pretty high end stuff and *very* costly. Now they might check for dry eye, but even then *only* if someone noticed some possible symptom of it.
They don't try to give an exhaustive "let's find EVERYTHING that might **ever** go wrong with this dog NOW!" kind of exam. They aren't looking for huge problems to treat -- and frankly there's only so much of that you *can* find anyway. And to then try to make them responsible? No - that's not fair nor reasonable.
You aren't getting a dog with any guarantee. They'll tell you, usually, if there is some obvious thing wrong, but that's only if someone is switched on enough to *see* a problem. (Like eyes that are red or goopy or puffy or swollen because of dryness ... but a deep eye exam? Shoot -- in all my years as a dog owner I've only had one dog have an exam like that and they were looking for something off the wall then!!)
That being said -- to address your other questions:
Frankly, a dog's eyes are their *weakest* sense. Their vision is essentially poor anyway. Night vision usually is better than day vision. But dogs really don't rely as much on seeing as we do.
Their eyes are more on the side of their heads, rather than both at the front of the face like ours. So their depth perception sucks anyway. (Cockers not quite as badly as a breed with a narrow face like a sighthound or collie where the eyes are positioned truly at the 'side' of the head almost.)
They rely on scent and sound way more than vision. And scent really more than anything. And a dog that goes gradually blind? In total honestly? He won't really even notice!
If you are forward thinking and train him with both touch and sound -- realizing that he will eventually BE blind ... and his hearing may decrease as he ages, so get him used to following touch signal and SCENT. He'll be so prepared for blindness it won't slow him down hardly at all (IF at all).
If you do some scent training with him -- you'll be amazed -- as his vision decreases, his scenting abilities will soar! When we had Muffin the Intrepid, he had no ears -- literally his cocker ears were SO bad, we had to have his ears removed. STONE 100% completely, fully deaf.
so? He learned sign. Shoot, he read lips better than most humans. BUT ... that wasn't even his big deal. HE became, **after** losing his hearing, our **watchdog**. No joke -- he was NEVER wrong. And the other two dogs relied completely on HIM. Because his nose became so incredibly astute, to make up for the loss of hearing, that he was absolutely infallible.
One day he went running into the bedroom -- gave a big "BOOF" and came back out to get me. I knew it wasn't anything 'threatening' because he wasn't freaked .. but man, there was something THERE he wanted to show me.
Shoot -- he sat and 'pointed' with his nose -- not something we taught -- but he was staring SO intently **in the dark** at the window there was no doubt he was convinced something was outside! Shoot, my husband even went outside to look for an intruder ... nothing. We could not figure what the deal was and he's still locked in that incredible sitting "point" lest we forget WHERE this was.
Ok, finally I gave up -- I turned on the lights and lo and behold. THERE. Exactly where he was looking ... on the **outside** of the window (drapes closed by the way) was a freaking TREE FROG!
Remember -- he had been in the living room and went on a dead run to the bedroom? Yep -- he smelled that thing when it jumped on the window. He sure didn't hear it and he certainly didn't SEE it with the drapes closed. HE **sniffed** it.
And remember I said -- the ears are more important than the eyes! You won't even be able to tell he's 'blind' as long as you don't change furniture around to fool him.
Now surgery would be *great*. BUT ... I'd ask, first off, what the long term results would be. Will the cataracts grow back? Is this going to increase the chance of infection or dry eye?
I'm not one to make them suffer -- (remember, I'm the one spending a ton of money to drive 1200 miles to 'rescue' a sick puppy! we're more than willing to spend money!) BUT only if it's really something that will gain something for them. If that surgery is pretty well guaranteed to buy him 10 or more years of vision -- that's one thing. But if they think the cataracts could recur in a couple of years? Hmmmm -- be careful. Don't bankrupt yourself for something pretty easily trained.
Ask yourself what will truly be better. You'll have to *deal* with his blindness. You'll have to make sure no one "blindsides" him -- sneaking up on him to spook him (some folks/some kids think it's lots of fun to sneak up on the blind dog to make him jump and YES, people can be like that). But if you take the time to train him to scent ... shoot, take him to tracking classes and let him EXCEL at it. If that organ that *has* to become stronger anyway, becomes trained to be absolutely ~excellent~ you will build his self-confidence in a big way.
I don't agree with your husband -- that he's "just a dog" or "not a child". He is a responsibility you took on. BUT, at the same time neither is it worth financially bankrupting yourselves when you may need the money to actually save his life.
What if ... in a few years ... he needs serious money spent on surgery for a luxating patella or some accident causes a problem? Will you feel like you've already spent the 'limit' on this dog because you went into hock for his vision, so there won't be money *later* for a life-threatening thing?
That's not MY call. That's yours. You know your own spouse -- but I'd quiz the vet thoroughly on this -- find out the cost of the drops (if they are for dry eyes they *are* pricey!) and factor that in.
There are good rescues and bad ones. But ANY rescue is always low on funds. That $250 or $300 they may charge for a dog is *not* all inclusive and they don't make money. But there are absolutely no 'guarantees' -- simply because most health issues are incredibly difficult to foresee even when you are looking for them!