That's what I think ended up happening. I remember being so sad Summer Night City wasn't included on the final product after hearing months before it was one of the tracks! No idea if the two tracks were recorded or even existed as demos though.
Not an album as such-but a 4 track celebration of ABBA
They were very busy during those ABBA-Esque days, so I could see them focusing their efforts on the 4 songs they picked (instead of recording demos for many). Since they had already done Gimme! Gimme! Gimme!, it's easy to imagine they had already talked about other tracks that would make great covers and never needed to demo any others. (??) Just speculation on my part. If no demos ever turn up, wouldn't an ABBA-Esque 2 be a wonderful treat? Perhaps Andy's live ad-libs will be all we ever get.
Andy only said recently they don't want to go near covers and will only focus on their own work. So very unlikely to see a new EP! In a recent interview he seemed really keen to move very far away from Other People's Songs, even suggesting some of the song choices were really poor.
If you remember, the original idea was an album of ABBA covers, never mind an EP. But I think you're right, they were so busy with touring and other plans, before the release, I think they shelved that idea quite early on.