In the first, you can imagine that the phrase is a shortened version of … the amount of caffeine (that is) consumed would be variable … It’s correct and natural. ‘consumed caffeine’ on the other hand would only be natural in a context where there was a comparison with some other kind of caffeine, such as ‘unconsumed caffeine’

There's not much difference. 'The amount of caffeine consumed...' vs 'The amount of consumed caffeine...' have the same meaning. The first version sounds smoother, and my preference would be to write it that way. There is an implied 'that was/has been' that's nested between caffeine and consumed, i.e. it is equivalent to: 'The amount of caffeine that was/has been consumed...' It's a general construction: 'The number of children born in a year determines the annual birthrate.' 'The amount of ore mined varies at each location and over the lifespan of the mine.'

There is no difference. Adjectives normally go in front of the nouns they modify. However, participles are an exception to this rule. They can go on either side of the noun. Sometimes they MUST be placed after the noun to serve as an anchor to a clause. This applies to both past and present participles. Here are some examples: "The amount of consumed coffee was surprising" "The amount of coffee consumed was surprising" "The amount of coffee consumed by the students was surprising" In the third example, "consumed" is the anchor for the adjectival clause "consumed by the students", so it MUST follow "coffee". The same applies to present participles: "Look at the singing boy" "Look at the boy singing" "Look at the boy singing the song" In the third example, "singing" is the anchor for the adjectival clause "singing the song", so it MUST follow "boy".