I’m not a specialist, but let me show my fearless explanation. I think two sentences have different constructions but both have the same meaning.
When it comes to their constructions, I would put them this way.
誰が盗みましたか？（ask for information）
→誰が盗んだ（change the form in order to implant in an indirect question）
誰が盗んだのですか？（ask for an explanation）...1)
→ 誰が盗んだのだ（change the form in order to implant in an indirect question）
1) I agree with your guess that the 「の」 particle is for explanation.(at this stage)
You say that someone said that 「のか」sentence has more emphasis on the verb. I can understand his feeling to some extent. He might feel that way maybe because 「のだ」is used to express the speaker’s strong opinion or decision in some cases. But I would take「の」as a particle for explanation rather than for emphasis, in this case.
2) At this stage, I would take「のか」as a tool to implant the first direct question 「のですか」into the final stage. I think it's similar to the function of 'that' conjunction in English.
Anyway, I would use both of them with little awareness of the difference. Also, if someone asks me those questions, my answer will be the same. It might be better to think they’re the same and「の」exists only in order to express the construction.
EDIT: I added 1) 2) to my answer. I'm very sorry to confuse you. But I hope what I'm saying will still be your help.