Thanks TheRealCanadian! As always, people like you, Jackolantern, Nelsona and others always inspire and reinstate partially-shatterred hopes in desperate peoples' minds who are experiencing tricky situations like mine. I also took it with a grain what this guy on the phone said to me because based on his attitude he was not very friendly and conversed in a condescending tone with me. He stepped out for a few mins to consult with someone else, but this phrase made me a bit alarmed: "Why are you applying for her now, not back in 2010? If this case is on my desk, I would ask the same question".
Actually you are probably right, there should be no statutory time period during which the derivative child can join the principal as long as the relationship exists, the principal is alive and the child is under 21. This is what I found online:
The term “following to join,” as used in . . . INA 203(d), permits an alien to obtain a[n] . . . immigrant visa (IV) and the priority date
of the principal alien as long as the alien following to join has the required relationship with the principal alien. There is no
statutory time period during which the following to join alien must apply for a visa and seek admission into the United States.
However, if the principal has died or lost status, or the relationship between the principal and derivative has been terminated,
there is no longer a basis to following to join. As an example, a person would no longer qualify as a child “following to join” upon
reaching the age of 21 years (unless they qualify for the benefits of the Child Status Protection Act) or by entering into a
marriage. There is no requirement that the “following to join” alien must take up residence with the principal alien in order to
qualify for the visa. (See 9 FAM 42.42 N11.) The term “following to join,” also applies to a spouse or child following to join a
principal alien who has adjusted status in the United States.
The cross-reference, 9 FAM § 42.42 N.11, “Derivative Status for Spouse or Child,” provides in part as follows:
a. A spouse or child acquired prior to the principal alien’s admission to the United States or the alien’s adjustment to legal
permanent resident (LPR) status, or a child born of a marriage, which existed prior to the principal alien’s admission, or
adjustment, who is following-to-join the principal alien, should be accorded derivative status under INA 203(d). No second
preference petition is required.