I think it worthwhile recalling that in the first few days, the search began in the South China Sea and the Mallaca Straights.
Whatever inquiries and coordination was done with countries for a northern search hypothesis has been either kept quiet (due to the issues and implications of the northern route on a variety of political fronts) or have been abandoned by a process of elimination.
The search efforts are not cheap. getting another government to search and having it turn into a wild goose chase can have political reprecussions of varying severity. Recall when the folks in Viet Nam packed up their kit and withdrew. Recall that India did likewise in a different area.
Yes, we the public do not know the whole story of the search effort. Do we have a need to?
James: thanks for posting that link. It adds little enlightenment, but does well explore the challenge of using the limited data set to establish a valid search datum.