Learn Finnish: Diplomaatin mukaan tilanne on vaarallinen. - According to the diplomat, the situation is dangerous.

Not always. While it frequently means 'according to' when used as a postposition with a genitive noun (like diplomaatin mukaan), it is also commonly used as an adverb meaning 'along' or 'with' (as in tule mukaan - 'come along').

Can I place mukaan before the noun, like a preposition?

No, mukaan acts strictly as a postposition here. You must place the noun in the genitive case first, followed by the postposition. Reversing the order to mukaan diplomaatin is grammatically incorrect.

I noticed diplomaatti loses a 't' to become diplomaatin. Do loanwords usually follow consonant gradation?

Generally, yes! Even though diplomaatti is an international loanword, it is fully adapted into the Finnish system. The double tt undergoes standard consonant gradation, weakening to a single t because the genitive -n closes the final syllable.

Is it mandatory to put the postpositional phrase diplomaatin mukaan at the beginning of the sentence?

It is not mandatory, as Finnish word order is relatively flexible. You could say Tilanne on vaarallinen diplomaatin mukaan. However, placing the source of the information at the beginning is the most natural and common phrasing, especially in news and formal contexts, because it sets the context for the claim that follows.