I think her post on social media is very odd - perhaps she was hoping for a public "whoopee, you are coming to visit!" response since she hasn't been there in 10 years for a social visit (just funeral and conference). I think you COULD (not necessarily SHOULD - that's important) have messaged her saying, "Let us know your plans" especially since she has hosted you quite a few times. Maybe she was hoping for a big response from everyone in the area, and she made plans with those who got the most excited.
Your husband called her, which is good - she's his sister. She maybe can "fit you in" or maybe not. So look, you guys aren't just that important to her or to her side of the family (no-shows for the graduation) OR they feel put upon because you visit them and perhaps they don't feel invited/welcome at your house. You say she hasn't come in 10 years, but not how often you've invited her.
I think you do have to shrug off her lack of interest in celebrating your son's graduation. That's a huge deal for you, and it should be for the grandparents I think, but if aunts and uncles don't want to come socialize with all his friends and your neighbors and everyone else they don't know, that's okay. Maybe not ideal, but not a huge affront. We didn't go 3000 miles for our nieces' graduations either.
I think you have to accept her the way she is - distant, and limited. She may want to see you when she comes, and that means you include her in something you have planned, or, if they are not essential plans, you reschedule those to allow for her. You don't cancel a party, but if you just planned to go to the park for the day with just your immediate family, you can scratch those plans to do something she will enjoy. She won't be back for another 10 years, if history is any indicator, so just enjoy it.
On the storage question, if her stuff is taking up a huge section of your basement, that's an imposition. If there are 2 boxes of stuff that you could easily ship or take with you in the car, then do it. At least she's leaving the door open by saying "next time you visit." But maybe she feels you owe her since she hosts you all the time? I don't know. I wouldn't focus on this.
I think you have to adjust your expectations of her and stop letting people hurt you who don't know any better. A lot of people have total estrangements from family members they never ever see. Just scale back and try to keep your emotions in check, for your own sake. "When people show you who they are, believe them."