I've read through.
It sounds like the previous owner bought three fluffy little piglets when they were all sweet and docile then when they reached the niggly age, she decided to call time on them.
It sounds like T has been picked on by the others since they reached maturity and things have come to a head.
D'you ever look at Gumtree and see the number of boar pairs that are "I;m selling my lovely boys, they are six months old and never bitten the kids" but probably knock seven shades of sh*t out of each other. Usually in a hamster cage 
I think you need to consider the piggie who's in the thick of it. He really doesn't sound happy.
The eye - if it;s a catarct , why on earth remove it? Unless it's a tumour, an abscess or an infection, the catarct is clouding of the cornea and lens.There are other underlying causes like back teeth and diabetes that can be ruled out.
If these pigs were mine (and I've had pigs as a child and now reliving Piggie Love when my daughter wanted some) , I'd seperate the bullied one and get him a piglet.
Matching him with another adult is tricky. A very little boar would soon catch up agewise (your boys are little) and have a 2+2 ( two pairs of boars)
(As a bit of history , if you want to plough through it. I'll keep it brief 
)
My DD (she's now 13) - when she was 9yo wanted a pet. I said 'piggies'
We got 2 boars Rescue (1yo brothers) she wanted boys. I learned very quickly they need space and room to do their own thing (We altered the playhouse for them but they didn't like their winter night cage)
When one died, we had a very dominant adult boar left. We Rescued a 6 week old piglet boar.
When the older one died we got two sows and had boar neutered (kept apart till he was sterile)
He died and we have the sows.
Neutering is not without risk, my boar was a one year old good solid boy. It does mean without the bollocks they don;t get impaction (go on, Google it) but I wouldn't do it just for that.
If you get a tiny piglet boar (get a Rescue to bond them) then he'll accept him, though, there will be the time when the piglet reaches adulthood and your boar might find himself ousted. But (as happened with our piglet and old boar) once they're established , the bossy one will take charge.
It's not guarenteed that they'll be harmonious.
Or- you could neuter and pair him with a sow. Keep them well away from the boars to stop them fighting. Tricky though.
I'm actually quite at the previous owner TBH. If I was recommending a piggie set up for someone who'd never kept pigs, it wouldn't be 3 fighting teenage boars, one with a gammy eye. Oh and BTW , have'em done 
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She's pretty well passed on the problem to you