I think I understand that sustaining and supporting means submitting.
I would say that "full" support and sustaining, theoretically, "involves" an element of submission, absolutely, but I disagree completely that it "means" submitting.
I have offered my honest input in every calling in which I have served in my life, even when I disagreed with something that was being said or proposed. I remember distinctly one occasion where I mentioned to my Bishop after Sacrament Meeting a concern I had as a result of something one of the speakers had said in a talk (a relatively minor thing, but something that is important to me). He thanked me sincerely, because it hadn't registered that what was said might be interpreted the way I saw it potentially being interpreted by some people.
I went to him and told him my concern specifically because I "support and sustain" him - and I view sharing what I shared as critical to my effort to do so. Withholding something of importance is not sustaining or supporting, in my opinion; in fact, I see it as the opposite of sustaining and supporting - or only sustaining and supporting in a watered down way.
I always try to express disagreement humbly and meekly, and every leader I have had has been grateful for my honest input and the way I have given it.
On the other hand, the idea that we have to accept and follow leaders' demands even when we feel they are wrong is one of the most repellent ideas I have encountered in the Church - and lots of other places, as well. It's Lucifer's plan, in a nutshell, and I abhor it.
I am fine with accepting a leader's decision with which I disagree if I feel it isn't causing extreme harm (after stating my opinion in the most appropriate setting), but I have no problem with opposing something I feel is wrong and causing serious harm.