How do は and もの work in this sentence?

Question

Consider the following bolded sentence from Fate／stay night (previous and next 2 sentences provided for some context).

古来より、魂というものは扱いが難しい。 在るとされ、魔術において必要な要素と言われているが、魂《それ》を確立させた魔術師は一人しかいない程だ。

魂はあくまで“内容を調べるモノ”“器に移し替えるモノ”に留まる。 それを抜き出すだけでは飽きたらず、一つの箇所に集めるという事は理解不能だ。 だって、そんな変換不可能なエネルギーを集めたところで魔術師には使い道がない。

Treating もの as a nominalizer makes it look like "The soul is, at the utmost, something you'd study the substance with [魂は(で)内容を調べるもの], something you'd put inside a vessel [魂は（を）器に移し替えるもの], however this looks quite dubious - can you put the same topic particle は to two different uses in the same sentence? Is there a way to employ は in one way that would make sense in both nominalized clauses?

In other words, I can't make heads or tails with the grammar here. How does this sentence work, and what does it mean?

Answer 1

The sentence in question basically says that the soul is like liquid, and must be always inside some kind of container. A magician can't drain soul from someone and keep it on its own.

You seem to have failed to translate the verb 留まる (=stay, reside) at the last. The basic structure of the sentence is "魂はあくまで～に留まる" (The soul absolutely stays in ～). And the rest describes where the soul must stay.

Taking the context into account, the remaining two elements seem to correspond to the source and the destination of the soul, respectively. So I would interpret them like this:

• "内容を調べるモノ" - The thing which (a magician) investigates
• "器に移し替えるモノ" - The thing/vessel into which (a magician) transfers the soul

Here モノ is not a nominalizer but a simple noun meaning "thing".

However, the second one is puzzling to me, because the modified noun is repeated in the relative clause. It's as odd as "The thing into which you transfer the soul into a vessel", "The book which I read the book" or something. I feel this should have been "それ(魂)を移し替えるモノ" or just "移し替えるモノ". I may be wrong, but I can't think of the better interpretation.