Learn Finnish: Tulkki on mukana valtuuskunnassa. - The interpreter is along in the delegation.

The comitative case (like valtuuskuntineen) is quite formal and often implies a permanent, tight association or ownership. In this sentence, the interpreter is simply accompanying or included in the group, so everyday Finnish uses the adverb mukana (along) alongside the inessive case (valtuuskunnassa, in the delegation).

I usually see mukana used with a possessive suffix, like in minun mukanani (with me). Why doesn't it have one here?

When it takes a possessive suffix, mukana acts as a postposition meaning 'with' a specific person. Here, it is acting as a standalone adverb meaning 'along' or 'involved'. The interpreter is not 'with' a specific person in this phrasing; they are simply 'along' as part of the delegation.

What is the base word for valtuuskunnassa, and why does the middle change?

The base word is valtuuskunta (delegation). It is a compound of valtuus (authority) and kunta (group or municipality). When you add the inessive ending -ssa (in), it closes the final syllable. This triggers consonant gradation, changing the strong nt to the weak nn.

Could we just use the postposition kanssa instead to say 'with the delegation'?

Yes, but the sentence structure would change slightly to Tulkki on valtuuskunnan kanssa (The interpreter is with the delegation). However, the original sentence using mukana valtuuskunnassa is much better for emphasizing that the interpreter is actively included in the traveling or working group, rather than just standing next to them.