"I sustain President Nelson" amounts to "I acknowledge that Russel M. Nelson is the president of the Church of Jesus Christ of Latter Day Saints, having been chosen by God Himself to perform that role." It is not an unconditional show of support for any and all words or actions that he undertakes, it is a promise that I will support him in performing his role, and not (necessarily) at other times.
It's like the Article of Faith about being subject to kings, rulers, and so forth in honoring, obeying, and sustaining the law. That doesn't mean that some sentence that Joseph Smith wrote in some letter has the power to nullify the entire rest of the Gospel because "we obey the l@w!!1!", scriptural history is in fact replete with examples to the contrary. It means we support legitimate government leaders in legitimate government pursuits. Illegitimate figures, such as... Well, most of them today in America and probably most other countries, such as Brazil or California. These are not to be sustained. By definition, they are executing treason against their countries.
A bad but still legitimate leader, by contrast, is to be sustained. Eli was once the high priest, and his sons sucked, but they all were still the legitimate ecclesiastical leaders of the legitimate Church directed by Jehovah. If he does wrong or clearly dishonest things, those are not to be supported and may be called out, but that is not the same as "not sustaining the individual". Voicing support for what they perform correctly and praying for their wisdom are good things to do.
/gospelaccordingtojames,nonotthatjames
