That sounds like a good idea, why did I not think of that myself.   I still don't think the seed will show imported keys in Electron cash (why else would it have given me that warning) but at least I could try. Thanks!

But it would be a bit of a logistical nightmare. Could I not simply enter the keys into Electron Cash just like I did in Electrum, or is that not possible? I have both my public and private keys.