Many thanks again for getting back to me, it is much appreciated.
I tried Hiveworld and I also tried https://hivetasks.com/account-recovery. Both continued to return RPC errors, just at different stages of the process.
However, thanks to @deathwing, I've now got another way in which has worked for recovering the above-mentioned test accounts and which should therefore work on the day of my 9-min window. This involved installing Python and broadcasting the transaction myself. Deathwing walked me through the process and we tested it together - it works! I'm utterly delighted and a bit nervous too. I'll breathe again normally after the 31st.
I'll let you know how it goes. Thank you so very much for your help with this 🙏
I wish you luck. Python is definitely the best solution, but definitely want to set up and test that first.
I'm back and utterly delighted. I had tested the script a number of times and it ran smoothly on the day (today). Thank you very much again for your advice and the excellent guide you put together 🤗