I wish I were going to be there when the trunk gets opened. No telling what's in there. I interviewed a WWII vet once who had some amazing souvenirs from his time in France. But I also think it's incredibly symbolic that your father literally kept things locked up. When I interview veterans, I always discuss ahead of time whether they want to share any of their war experiences. About half of them don't want to discuss the experience in depth and those that do, usually did not lose anyone close to them during the war and were not injured themselves.
But some are giving accounts who never had earlier. I am usually hired by the children of those I interview and often they are surprised that their father opened up during the interview about their war experiences. I guess these WW II vets understand that if they don't tell of their experiences now, they never will. [This is where my counseling background is helpful. It's obviously a delicate issue.]
Regarding suggestions, just be aware that even though this period of your mother's life was decades ago, it can be very emotional for her. It's nice that you will be there when she opens it up. It could contain letters from her to your dad during the war etc. and those will be powerful memory evokers. I would also recommend that you open the trunk in the morning when you and she have lots of time to discuss her feelings etc and not do it at night where she'll likely find herself staying awake processing things on her own.
And one more suggestion: Don't throw anything away that's inside even if you don't think it has any value. It's probably worth something to someone in your family at the very least. And if no one in the family is interested, ebay is full of people that have interest in WWII memorabilia...even little things.
It's wonderful that your mother likes to write. It would be great if she were to answer the questions you suggest and then follow that up with her feeling after it's been opened.
I'd love to hear how things go. Best to you and your mom.
--Bridget