It should be correct, report it! :)

誰在你媽媽的前面？ is now accepted.

Question words like 谁, 什么, 哪儿, etc., stay in the position that would be occupied by the answer. So, for example, say the answer to this question is 李老师. The statement would be 李老师在我的妈妈的前面, so the 谁 just replaces the "李老师" at the beginning.

Could be, but you have to make the sentence more complicated—那个…是谁

Generally you should always keep the last 的 in the chain. That is also the case with this example sentence.