In the context of restaurants, convenience stores and similar situations, is it ok for a customer to ask for something that was said in keigo (or in manual keigo) to be repeated in more "normal" Japanese? Or would staff rather speak in English rather than non-keigo Japanese?
Assume the customer is not asking because they want to be more intimate, but because the customer (an obviously non-native speaker) hasn't learnt Japanese in its entirety yet.
Sure, that's fine. You could say 敬語{けいご}は難{むずか}しいので、タメ語{ご}でしゃべっていただけますか？ Although it's a bit funny when a beginner uses the word タメ語 because it's a bit slangy. But I can't think of a better way of saying it.
簡単{かんたん}な日本語{にほんご}でおねがいします should work well too (and has no slangs).
簡単な日本語... is okay.
I think it is OK. I used to do it a lot, but the staff may have trouble not using keigo. It works well at a CD shop, not that well at the bank…
Also, it's quite impossible to be satisfied over the phone. Indeed, in a shop, the boss will see you're a foreigner having trouble, and maybe let his staff use less formal wordings. However, in a call center, the boss will only see/hear his staff speak badly on the phone, and scold her (hurrah for jobs for women!) harshly later I think…
