Very difficult DIL

It is a pity your dil does not have better manners but I agree with your husband. You can not force her to be what she is not so it is better to ignore her behaviour. I can quite understand why you find her behaviour disrespectful.

Give her space she's not obliged to either like you or spend time with you.She married your son ,not his parents .She's entitled to live her life her way ,she's not stopping her husband and children from seeing you so there isn't a problem.Respect has to be earned in my experience,I got none from my FIL for years so he had minimum effort from me and only that because I loved my husband who loved his dad.FIL didn't like me from day one and it was very clear ,maybe your DIL feels that way about you.

Leave her as she is. Remain courteous and offer hospitality but don’t press her on it.
It is a good thing she feels able to go and read a book/ catch up in sleep or whatever when they are staying over. As long as she is able to “escape” she may well tolerate visits.
In observing other families I have noticed that this sort of behaviour is tolerated much less in DILs than in SILs, people don’t seem to mind their son-in-laws reading a newspaper or disappearing off to do x or y, but the expectation is that women will be present and amenable.
Enjoy the fact that you get plenty of time to chat with your son.

I don’t see anything wrong with what she has done.

She hasn’t Skyped you? She doesn’t need to. Maybe she’s taking that time to finally have a breather.

Those 1-2 hours? Probably stopping her from snapping.
3 kids and guests sounds like hell and I would go for a break as well.
She married your son. Because they worked well as a couple.
She isn’t obligated to maintain any of your expectations.

Your husband is right if she’s not comfortable with you it will only get worse if you don’t ignore it

I think your husband is right. Leave her to it otherwise you just create a difficult atmosphere. Enjoy the time with your son and granddaughters.