I don't see any decline in LA. Likely for every person leaving, five more are coming in from across the border. Doubtful those people are taking the census poll.
You are surely correct. This census study only studied flows of residents within the country. International migration numbers were not considered. The point is that this is what those who have lived here for some time have chosed to do.
Actually, you'd be surprised - the vast majority do respond.
Place of birth, where you lived last year, how long ago you entered the US, and whether you're a citizen are all asked (legal or illegal residency is not asked about.)