Is dismissal of an theory on the basis of implausibility committing the fallacy of Argument from Incredulity?

Is dismissal of an theory on the basis of implausibility committing the fallacy of Argument from Incredulity?

Let's parse two alternative forms of the argument.

Form A

P) X is implausible

C) Therefore, X is impossible 

OR

C) Therefore, X is false

This is a fallacy (reverse appeal to possibility). Implausible and unlikely things happen all the time (e.g. Trump winning the 2016 election). Thus, something having a low probability does not mean it cannot or does not occur.

Now consider:

Form B

P) X is implausible

P) Very little (if any) evidence is provided for X

C) Therefore, we should suspend belief in X until there is more evidence

This is far more reasonable and is consistent with the principle of parsimony. Due to the conjunction effect, the more claims added to a theory, the less likely it becomes, which in turn demands a higher standard of evidence. Assuming there are theories/explanations that make less complex assumptions, these theories are to be preferred over X unless more evidence in favour of X is produced.

The problem with most conspiracy theories is that the claims they make are implausible (given that simpler explanations exist) and the conspiracy theorists rarely provide suitable evidence to make their statistically-implausible theories preferable to simpler ones.

Basically, I agree - if there is evidence, it should be evaluated. Ignoring the evidence to focus on pure probability is fallacious. But if insufficient or no evidence is given, dismissing extremely unlikely claims in favour of simpler ones is not fallacious at all.

The Argument from Incredulity boils down to this: “I cannot imagine how this could be true, therefore it must be false.” It is a fallacy because the fact one does not believe the proposition does not make the proposition false. Truth or falsity is independent of belief. For example, you can believe with all your heart and soul that the earth is flat, but that belief does not change what the earth actually is.

 The error, as I see it, is the focus of the discussion. Instead of saying, “I do not believe it, therefore it is false,” the response should be, “I do not believe it. Please provide more evidence.” 

 Always keep in mind the burden of proof rests with he who makes the proposition. 

I think that the stakes should be part of your decision making. For example, if it's implausible that a comet will strike a house, the owner should stay. The stakes of leaving are high, like the cost of moving. This dismisses the theory of a comet striking the owner's house on the basis of implausibility and I argue that it's warranted. 

On the other hand, not dismissing the theory that a comet will not strike the house is also warranted because the stakes of staying are arguably low if we attach a really small probability and do some expected value calculation (if I recall this stuff correctly).

I do understand your point though. A detective might want to dismiss some implausible explanations of a crime scene while keeping others in the background. This might be assessed through experience.

Example: It is implausible that the crime is staged. But later on, the detective finds out about this kid that loves to play pranks. And this kid lives were the "crime" occurred. The implausible theory of a staged crime might be taken noted. 

Your question might be connected to the inference from the best explanation.

I think that the argument from incredulity would go something like this:

Witness: Detective! The kid staged the crime!

Detective: That's impossible! I will dismiss this theory.

If the detective dismissed the staged crime theory on the basis of incredulity, instead of on the basis of limited resources or something, then the detective will be committing the fallacy of argument from incredulity.