This invitation is bizarre. I don't understand inviting you to something and then announcing that the tickets are already purchased, but MAYBE you could get a spot somewhere near them. I would think if she wanted to reconnect, there would be an effort to see if you can go and THEN buy the seats together, or purchase a third seat thinking you probably could go, and either eat the cost or invite someone else if you couldn't go, or even sell the spare to a stranger. This is one of those chances you take. "Let's buy a third for Meliboea, just in case. She loves this stuff, and if she can't go, probably Roger can, or will see about a refund or Craigslist."
Given your work and health issues, I think it would be fine if you bailed. Sometimes life takes us on unexpected turns. You're not thrilled with the lackadaisical approach either, and the A-list, B-list mentality and obligation. You're willing to eat the cost of the ticket, so neither Sally or Kate are out any money, and you weren't a priority of sitting with them in the first place, so they'll be fine on their own.
I might offer the ticket for them to sell to another friend - "If you know someone who wants to go, the ticket was $XX if they want to buy it from me," or they (one or both) can buy it from you and sell it to this other friend, or even sell it to a stranger. The logistics of this is getting the money from the sale, so it would probably be best if Sally or Kate purchased the ticket and were reimbursed, since 4+ years later, you probably don't know this friend. On that, don't feel obligated to cover the cost of this ticket if they can't sell it. This would be their choice to buy it, and they would need to be willing to eat the cost if no one takes them up on it, just like you are willing to eat the cost. <--That kind of strayed, but just a thought. I would want to see about getting my money back, if I could, even if I myself list an ad.
I think your life circumstances leave you in the clear to cancel if you need to, and I don't think you're being over-sensitive. You were invited as an afterthought with no efforts to keep you within the group, so the whole situation is just not setting well overall.