I think you are confusing countries with investigatory body's. Countries do not investigate aviation incidents, they have an independent body which does it for the precise reason you are outlining above, it is almost irrelevant which 'country' is actually taking part. In the UK we have the CAA who are directly accountable to the CAA and if they were invited to take part they would, that decision would have nothing to do with the UK or government so to speak.
If anything the fact this incident is being investigated by so many effectively guarantees impartiality because they are all independent organisations with their own expertises and competences. It would be suspect if Ukraine was investigating this alone but they aren't and have involved the entire community.
As for the US link well I am glad and would expect the US to be involved to the greatest degree possible given an American company built the plane and also their NTSB is the leading authority in investigating aviation incidents worldwide.