Oh, the poor little guy! I'd have to agree with you that abuse is likely part of Makis' history.
I also agree with you VERY STRONGLY that subjecting him to loud noises would be really upsetting to him, and would not achieve the desired results. If she does that, then Makis might start associating the loud and traumatic noises with her -- and that would make it even harder for them to build a close and trusting relationship!
Has Grandma had Makis to the vet to be examined, and to discuss this problem? That would be a good first step.
My vet -- and many, many of my PT friends -- recommended a product called Feliway. It is a synthetic pheromone that comes in a spray or a plug-in diffuser, and helps cats feel calmer and more comfortable. It made a big difference in my multi-cat household in just a matter of few days, and I think it would definitely be worth a try in Makis' case.
Now, having said all that, there are cats who just tend to be timid and not "people" animals -- and that might be Makis' basic personality. However, I'm sure his vet will be able to help Grandma come up with some strategies to help him be happier and better adjusted. God bless her for adopting from a rescue shelter -- and the best of luck to all of you!
Hugs,
Diana
P.S. Here are a couple of links with further advice about dealing with timid cats -- hope they help!
http://www.gardenstew.com/about92.html
http://www.best-cat-art.com/nervous-cats.html
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