I’ll just say right at the start OP that I don’t think you’re being unreasonable. Not at all.
However, I suspect it’s the social media that’s the main issue.
My situation bears no resemblance to yours at all but for various reasons, I struggle to keep up with messaging at times. There are times when I want to share something on social media but I don’t because I’m aware that I haven’t yet replied to a friend and they might see the post and potentially be upset.
For clarity, the messages aren’t urgent, they’re just very long, chatty messages that require the same type of response - and sometimes I just don’t have that in me. In contrast, posting on social media is quick and detached, you don’t need to put any emotional investment into it.
I’ve found people fall into different camps. Some people always manage to message, no matter what. They’re very much of the belief that “a text only takes 30 seconds to send.” Then there’s the second camp of people who sometimes find the constant demands/expectations of messaging overwhelming, and can’t always summon up the emotional energy to “just send a message”. When life is tough, this second group of people tend to retreat and don’t seek out personal connections from their wider circle of friends.
People in the first group don’t tend to see the viewpoint of people in the second group. They resolutely believe that if you have time to post on social media then you had time to send them a “quick message”.
My good friends all fall into group 2. Sometimes they are slow to message me too but they might share a petition online or upload a photo. I understand that the mental space needed to text and message is very different to what’s needed to post on social media. We are all very forgiving of each other’s difficulties with communication but equally we know that if something was needed, we’d be there in a flash.
I have a few more casual friends in group 1. They are the reason I would hesitate to post something online, because I know they’d feel annoyed if I’d “prioritised” posting on social media above replying to them.
I think your friend falls into group 1. But I think it’s more than that - she wants to be part of the drama and she wants insider gossip so she has bragging rights. Just look at how she’s pointed out the fact she gave you money/gifts. It’s all about her and getting what she thinks is appropriate praise, and being included. She’s miffed because you haven’t made her feel special. Posting on social media without messaging her separately when she’s Done So Much to support you is your “crime” here I’d suspect.
I don’t think you should let her suck up your valuable emotional energy right now. It’s unbelievable that she’d send you a message like that. Truly, truly unbelievable. Do you care about the friendship or can you let it go?
I hope treatment goes well for your beautiful boy 💐