Let's look at the Wikipedia definitions first.
Fake news is false or misleading information presented as news. It often has the aim of damaging the reputation of a person or entity, or making money through advertising revenue. Source: Wikipedia
Dissent is an opinion, philosophy or sentiment of non-agreement or opposition to a prevailing idea or policy enforced by a government, political party or other entity or individual in a capacity of contextual authority. Source: Wikipedia
The key difference is that fake news is 'information presented as news' while dissent is described as an opinion or philosophy. As such, the distinction between the two terms is not necessarily the intent of the author but the way the information is presented.
I would posit that content that clearly presents itself as an opinion of the author is generally not fake news. Of course someone might be misrepresenting themselves, but that's better described by the term 'lying' than 'fake news', in my opinion.
If something is presented as truthful information about definite known facts, but the journalists do not have evidence that reasonably supports that certainty, then you get into 'fake news' territory. This can range from verifiably proven false information, to complete fabrications, or even to just grossly exaggerating the evidence. An article that takes something heard as a rumor and, with no further investigation, presents it as clear and undisputed truth, is 'fake news'. Still, falsehoods don't have to be fake news, they may be mistakes which are later retracted.
Intent also plays some role, for example satire may be presenting falsehoods as truthful information for comedic purposes even though that's not considered fake news.
A publisher may hire writers based on the knowledge that they privately believe in conspiracy theories that may further the publisher's cause. Or they may hire freelance journalists to promote certain view points and make it look as if it's coming from some independent journalist. Whether that constitutes fake news depends in part on how the information is presented to their audience. Does undisclosed affiliation immediately mean it's fake news? I don't think so, but it's certainly a grey area where deception meets fake news.
To sum up, I think the two relevant factors when considering fake news are:
How is the content presented, does it claim to be truthful information or is it presented as an opinion?
(from your question) How does the content adhere to "journalistic best practices and principles of scientific inquiry"? If there a lot of effort to deceive readers about the origins of the content then you get closer into fake news territory.
As a hypothetical, suppose the mainstream news media in my country have pretty much all agreed that there is enough research showing that Chemical X is a danger to the environment that they are now reporting it as fact. I have honestly investigated this research and found it flawed and also done my own research which indicates that the chemical is not actually that harmful. I Tweet my findings and am promptly flagged for fake news because my findings contradict the generally accepted facts. Am I a fake news peddler or a dissident?
If your tweet is supported by your own research that you believe to have been conducted correctly, then I would say it's not fake news. It may be wrong despite your best efforts, but it's not fake news as long as you present the information (that the mainstream conclusion is wrong) as being the result of your own research which you also provide (maybe as a link). Others may disagree with your research, perhaps because they disagree with the way you conducted the research. If that's the case then they may label it fake news, after all that's their opinion of your research.
As you can see, it's quite subjective where the line can be between fake news, a genuine mistake (on your part) or being labelled fake news incorrectly (because your research turns out to be right). In your hypothetical, Twitter is the one to determine if they allow your tweet on their platform. That's not so much an issue of whether your report is right, it's about Twitter having the final say over content on their platform.
If you were publishing the information elsewhere, it may have to be settled in court. Generally speaking, I think your claim would be allowed by free speech. On the other hand, there may be limitations, for example when your statements hurt someone or some company (maybe the manufacturer of that Chemical X) who decides to sue.