You're fine OP. Just ignore.
It's an incredibly difficult situation. You can't help someone who refuses to help themselves.
I do hold hope that when she does come to visit, seeing her Grandchildren in person and being around people will help her immensely.
So I think you have to park the last 6mths - go into it with a clear head and just see how it goes.
Maybe practice some holding the boundaries phrases - e.g 'Susan, not in front of the kids, ok?'. Little things like that and if she refuses - have your car keys to hand. Have a 'go bag' ready. Not knowing how old your kids are but a bag put together that at a moments notice you can say - 'right kids we're off out - hopefully when we're back you'll have had time to pull yourself together Susan'. Then grab it and go - could be quite useful.
I would just play those 2 weeks with having an eye on always being in a position to leave. If you're the type of family who spend an hour in the morning downstairs in your pj's - maybe don't do that, maybe be dressed. So if it starts to kick off you can bow out with minimal fuss and just get the kids out with a trip to the park or something.
But at the same time - do leave some room for her to have her moments when the kids have gone to bed though. You have to strike a bit of a balance.
And actually it could be that you create some space to look back at some nice memories. Maybe get a photo album together, or put a little video. Or mention something fun e.g do you remember when we went to XYZ location and FIL did/said this funny thing. Oftentimes when people are grieving they actually want to talk about the person they are missing. Bringing him up in a positive way, that maybe brings a bit of a chuckle - can be really helpful.
So really my advice - be prepared for anything but go into it with an open heart. You may find that being around you, her son, her Grandkids she feels immensely better almost immediately, or there could be a couple of days of outpouring to begin with and then she might feel better. You're likely getting the snippet of the day when she's feeling her lowest and that's why she reaches for the phone - so to you being so far away, that's how it feels all the time. You might find in person, it's not quite as intense.
Also try and think of things to keep her busy - does she like cooking, gardening, dancing - maybe plan a couple of activities that might help her get back into a positive space.
I think you have to find a balance. If it's a nightmare - well it's 2 weeks, you know? For your DH's sake you probably need to grit your teeth through it. Make sure you've got some space for yourself in those 2 weeks as well. E.g book in a dinner with friends in the middle - so you can find a balance with it all.
You're not a nightmare OP, nor selfish or self-absorbed. I'd be feeling every bit as apprehensive if I were in your shoes.