Challenge informal appointment of Personal Representative

Absolutely.  As you may know from reading the other posts, when an individual petitions the probate court to be appointed Personal Representative of an estate under informal probate proceedings, she must provide notice to other interested parties.  As part of that notice, the candidate for Personal Representative must inform interested parties that they may petition the probate court to terminate the Personal Representative's powers and to begin formal proceedings instead. 

So, for example, under Massachusetts General Laws Chapter 190B, Section 3-414, the probate court may institute a formal proceeding for the appointment of a Personal Representative if an interested party raises an objection to the priority or qualifications of the current Personal Representative.  That section seems to address your question.

However, even if, as in your question, a person is not immediately notified of the informal proceedings, he will usually have time to file an objection with the probate court.  Under the new probate code, with very few exceptions, neither an informal nor formal proceeding may be commenced more than three years after the decedent's death.  However, a person who wishes to contest an informally probated will and ask the court to appoint a new Personal Representative has twelve months from the informal probate to do so. (See Massachusetts General Laws Chapter 190B, Section 3-108).  Hope that helps.