Do all of them simply mean "thing" or does it mean something else?
From my perspective, もの can have different meanings and what meaning you give it depends on the speaker / context of the sentence. With that being said, I think defaulting the meaning to "thing" is a good middle ground. もの usually seems to center around a singular object, entity, grouping, or etc of some sort from my experiences.
To illustrate this with your sentence above, let's go over the もの parts
Entire sentence for reference. 4 ものs total from what I see:
そして、かけがえのないものといわれる人生の価値は、たったの三時間でわかってしまうものであるようだ。もちろんその価値は、あくまで向こうが決めるものであって、普遍的なものではないのだろう。
- Here もの can mean thing, one thing, or etc.
かけがえのないものといわれる人生の価値
A.) The value of a human life, which is said to be a irreplaceable thing.
B.) The value of a human life is said to be one thing that is irreplaceable.
- Here もの can mean thing, everything, or etc
たったの三時間でわかってしまうものであるようだ
A.) A thing that seems to be understood (completely / in full) in just 3 hours.
B.) With only 3 hours, it seems like everything is understood (completely / in full).
- Here もの can mean thing, something, or etc
あくまで向こうが決めるものであって
A.) It's a thing, which is decided by another (group / person) in the end.
B.) It's something, which is decided by another (group / person) in the end.
- Here もの can mean thing, way, or etc
普遍的なものではないのだろう。
A.) Perhaps it's not a universal thing.
B.) There seems to be no universal way.
C.) Perhaps it's not universal.
Bonus: we don't even have to give a word meaning / translate もの for C. In this case もの could be seen as an emphasizing effect.
We can then take all of these parts and combine them to get an overall idea of what is being said / the meaning of this:
そして、かけがえのないものといわれる人生の価値は、たったの三時間でわかってしまうものであるようだ。もちろんその価値は、あくまで向こうが決めるものであって、普遍的なものではないのだろう。
And thus, the value of a human life is said to be one thing that is irreplaceable, a thing that seems to be understood (completely / in full) in just 3 hours. Certainly, this value is something that is decided by another (group / person) in the end and perhaps it's not universal.
You mention:
How do I know which usage is what next time? Because for me in 90% of those situations, context doesn't really help.
Context will always play some sort of factor, because at the end of the day you are translating here. And because of that, don't forget (it's an easy thing to forget) or become very comfortable with the fact that not everything has a 1 to 1 translation. Sometimes you have to choose what's best or makes the most sense depending on your target audience, which can include yourself, and the context, speakers involved, or etc. Also remember, it's only difficult now. This will get better over time as you come across more and more things and get used to translating
