I'm sorry to hear about your grandmother. I agree with squinks. You could always go now to be with the family, even if you have to leave and miss the service. It's the family that your grandmother left behind that will need comforting. A JW service is, as you know, a big sales pitch, not something that comforts. The focus won't be on your grandmother. Skip it if you have to.
And if the budget is tight right now, wait. Your grandfather will have all kinds of relatives and friends visiting him right now. But in a month or so, most of them will stop visiting, yet he'll still feel his loss. Just don't postpone it too long. I've heard of too many cases where one spouse dies and the other gives up.
BTW, it was great to meet you here in Texas, hope you had a great time!