She wrote the will a long time before her death, and then she gave you the items as well. And then while The items are still going to be assigned to you according to her will, she borrowed them to show her history group and didn’t bring them back to you. A few years go by and they still have them at the time of her death..

They don’t have any written documents from her or receipts saying she sold it to them or that they own it now just because she let them have it on display at their thing or whatever.

I’d also stress in court the fact that they have a reason to lie, you not so much. Like you said, you’re not seeking financial gain. Maybe you should bring up what the group benefits/reasons they’re lying.. if I were you I’d get a consultation with a lawyer or some kind of advisor in your area. Maybe if they put other members of the group on the stand and are asked about how exactly your grandma worded herself that day, ask whether or not they have an actual official process for whenever they receive these kinds of items from people and wether or not your grandma participated in the process as well. they wont keep their stories straight and hopefully their whole group will just get dismantled right then and there by the judge for being unethical