I have candle sticks for Jewish Sabbath celebration from my Grandmother. Not the same as wearing jewelry, but they are a mixed memory for me. I loved my Grandmother, but the religious use makes me uncomfortable.m
To me it's all about being honest with myself. I try to behave with honor and respect to myself. That's what I believe my Grandmother would want for me.
The candlesticks are where I can find them if I want to.

Rings can be resized and if my grandmother had left me across I would wear it. If my Uncle dick have left me jewelry I would have pawned it. In fact I need to get out the Silver Service he gave me as a wedding present and figure out where to sell it. Partially because he was a dick and mostly because no one needs a silver service unless they have a butler.

I wear a small whistle that came from a charm bracelet my mother had and a St Christopher the Navy gave my father in WWll on a chain around my neck. He wasn't religious, but it fits on a chain, it was his, and I don't feel weird about wearing it. As a matter of fact I felt weird the only time I left the house without that chain. I felt like I left a piece of me behind.

What a well written description of the dilemma.. From that alone, I’d trust your judgement ~

Do what makes YOU happy

It’s your grandmothers. She probably wore it with pride. She left it to you for whatever reason. I think you need to realise it’s not about you! It’s about her and it’s seems you are thinking that others won’t approve. Why on earth do think anyone would give a rat’s arse what you wear. What message do you think it sends out and whose business is it anyway? It’s weird that you have a problem with not respecting your grandmother’s bequest if you want to wear it!

I like the idea of adding on to the cross or star to make it something of your own. Nice to have a foundation of your grandmother's religion, but let it morph into something you believe in more fully now, or something iconic to your current life. Maybe a jewelry designer could help you out with an idea. I love the idea of feeling the embrace of your grandmother, by her jewelry, but it should also be on your terms.

I have a lot of my mother's and grandmother's jewelry and special pins, including a "Nixon's the one" pin. The jewelry box is where a lot of the stuff will stay, haha!

Absolutely not you honor them not the religion

I have a coptic cross from my godmother.
I feel the same way about my cross as you.
Maybe you could put a petite icon with it on the chain, like an atheist symbol, a satanic symbol, or a Micky Mouse? That way, it might not feel ostentatious.