如一對夫婦於外國合法註冊結婚，在香港該婚姻是否一定會被視為有效並可在入境處更改婚姻狀況並可依《已婚者地位條例》獲得已婚者的地位及權利? (例如:在荷蘭結婚的同志;在新澤西州結婚的父女;在伊斯蘭教國家結婚的一夫多妻)?

各國法律規定不同，互不干涉，於外國合法註冊結婚，在香港該婚姻不會被視為有效，入境處不能更改婚姻狀況，不能以《已婚者地位條例》獲得已婚者的地位及權利。
例如在荷蘭結婚的同志、在新澤西州結婚的父女、在伊斯蘭教國家結婚的一夫多妻等，進入他國都不被承認。

The answer is no.

In addition to being legally married, the marriage itself must also conform with all applicable laws related to marriage in Hong Kong.

None of the 3 marriages mentioned has met this requirement.

(For the record, the New Jersey marriage mentioned does not exist as a matter of New Jersey law. N.J. Rev. Stat. § 37:1-1(a) has explicitly prohibited such marriage.)

結婚不用到處分地方算吧，結過就結過， 只要把結婚照法定程序進行登記，而夫婦必需合乎當地法律，主要這對夫婦生活在那？
對不起紐澤西何時准父女結婚？ 鬼扯，我住紐澤西。

Are there any situations in New Jersey where incest would still be illegal?

Incest is legal in New Jersey "for people over 18, as long as it’s a consensual relationship,”

But restrictions apply, just as they do for couples with no blood relation. Sexual relations between any adult and anyone under the age of 16 are defined under New Jersey law as sexual assault or aggravated sexual assault.