The link above might help if you have the right iPhone adapter and the spreadsheet is a file on the passport. You can run and work on spreadsheets on an iPhone. iPhone display size is kind of an issue for me. With the right tv or a lightning to Hdmi adapter you can likely mirror your iPhone screen as an input choice or using airplay on your tv.
I assume tech support had you startup your laptop in internet recovery mode and had you run the disk utility. Without another Mac you wouldn’t have been able to try your laptop in target mode. Personally I’d also have tried a couple of different keystroke on startup options.
Ive never tried using internet recovery to install on an external, usb or thunderbolt, storage device. It could be that would be an option to get your laptop running.