Eyesight is thankfully the weakest of the guinea pig senses (it is our strongest in humans); piggies learn to compensate for its loss.
There is still a future and happiness for Melvin out there, whether it is a suitable female or male companion. You have saved his life and have made him happy beyond anything he could ever imagine. Everything more is just more decoration on an already iced cake.
Perhaps my Mali's story will help you?
When I adopted her sight unseen as an emergency to help clear an urgently needed rescue for a large intake of piggies in a very bad state, she was newly bereaved (her companion had been blind), she was 5 years old and had been surrendered with a very heavy heart by her owners not being allowed to bring pets with them when moving into sheltered accommodation. She had herself by them started to develop cataracts; the sight on one side was already about 80% gone. She'd failed to integrate into the residential old ladies rescue group. Bonding here was not quite easy but my master diplomat Terfel managed to trot happily after her or let her trot after him and allowed her to mount him all day while ever so gently and carefully taking over the reigns before she even noticed1. Once Mali realised that her two other female companions had the same eye problem (cataracts) as her, she dropped all reservations and happily joined in to become Tegyd's devoted sidekick. Her new piggy family gave her such a renewed zest of life that she lived for another 2 1/2 years to around her 8th birthday, happily creating new zooming scent loops until two days before her death; even though by that time she was so blind that she had to sollow her nose and her arthritis had turned her running into more of a waddle.
You can see a video of her buzzing around, aged about 7 years in this video here:
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My ever so skittish black Nosgan 'Nightsong' (because you never saw him and just heard him) who managed to bash in his eye on a grid the morning I was off to pick up a last minute added extra piggy surrender as a hopeful mate for him, so we had to drive up straight from the vet's to fundraising event with loads of homebaked Christmas biscuit tins in the back of the car so I could give him his 9 lots of eye meds he needed daily in an attempt to save his eye (thankfully we did and he even regained a modicum of sight). Anyway, he and little happy-go-lucky Nye who joined us on the way home from the Christmas rescye fair became a very attached yin-yang bachelor pair; another piggy that arrived here pretty much sight unseen...
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PS: Since Nosgan's death neutered Nye is happily married to my largest sow who'd refused any other boar I introduced her to. Since Nosgan didn't have much interest in the ladies and never got on with any, the boys had been living in the next door pen to big Hyfryd and her daughter Hirael for a number of months without any problems. The girls loved to flirt with a very appreciative Nye through the bars!
Nosgan And Nye Roaming Video
Just to show you that there is still a future for Melvin out there, even if it may not quite happen as you would expect; especially not this year. But something will surely come his way...