Regarding Revelation 22:19, does that only refer to the book of Revelation? I know many people who “cherry pick” the Bible: choosing to live according to "easier" parts of the Bible, while disregarding other parts saying they don't believe it's true. Does the judgment in this verse apply to them?
Revelation 22:18-19 is speaking directly about the book of Revelation, as the context suggests:
Rev. 22:18 I testify to everyone who hears the words of the prophecy of this book: if anyone adds to them, God will add to him the plagues which are written in this book;
Rev. 22:19 and if anyone takes away from the words of the book of this prophecy, God will take away his part from the tree of life and from the holy city, which are written in this book.
Jesus warns against those who would change "the words of the book of prophecy” referring to Revelation. However, since the Lord knew Revelation was the final book of scripture He would reveal to the world, then it stands to reason that His instructions were indirectly speaking about the entire canon of scripture, not just Revelation. In other words, to add or subtract anything to the final book of scripture is equivalent to adding or subtracting anything to the overall canon of scripture, and therefore no one may change anything in scripture now that the canon is closed.
Those who would knowingly counterfeit scripture are giving evidence of an unbelieving heart, and they will receive a harsh judgment according to scripture.
Regarding those who decide to obey or observe only parts of scripture, such individuals are also guilty of sin, but they are not necessarily in violation of Jesus’ warning in Revelation 22:18-19. The concern in Revelation 22:18-19 is for someone who intentionally changes or counterfeits scripture rather than for someone who merely ignores or disobeys it.
