Learn Swedish: Lars har dessvärre missat tåget igen. - Lars has unfortunately missed the train again.

Yes, they are synonyms meaning unfortunately or alas. Tyvärr is slightly more common in everyday speech, while dessvärre can sound a tiny bit more formal, but you will hear both used regularly in normal conversation.

Why is dessvärre placed right in the middle of the verbs har and missat?

In a Swedish main clause, sentence adverbs like dessvärre usually go in the mid-position. This means they are placed right after the finite (first) verb. Here, har is the first verb, so the adverb steps in before the second verb (missat).

If I start the sentence with Dessvärre, what happens to the word order?

You can definitely put it first for emphasis! Because of the V2 rule (the verb must stay in the second position), the verb har must come right after Dessvärre. The subject then gets pushed to the third spot, making it: Dessvärre har Lars missat tåget igen.

I know missa means to miss a bus or train. Can I use it if I miss a friend?

No, you can only use missa for missing a transport connection (like a train), an event, or a physical target. If you miss a person emotionally, you must use the verb sakna instead.