The new Runechild Sorcerer added spell list is supposed to be exchangeable and yet, those spells are locked within DNDBeyond functionality. Is there a workaround, or a change/update coming to fix this?

It’s the same workaround as in this video:

So the best choice available is to remake the whole subclass myself? That's an excessive amount of work for something they should be able to fix on the back end in less time.

No. You use the original subclass as a template and it automatically recreates the subclass for you. Then you just edit that one feature.