Are there grounds for reasoning that one who is naturally inclined to evil, and who yet acts morally is in some sense actually fulfilling the definition of the words, 'morality/ethics', and is therefore truly a moral person; as opposed to one who acts morally because his disposition is so, or because they are too timid to act immorally?
When you say disposition here, are you talking about emotional urges / demeanour or logical framework / worldview? There are at least four cases to consider here, depending on the combination of factors.
Case 1: Emotion-Go, Logic-Go.
In this case, a person holds both the demeanour and the worldview needed to act in selfish ways that harm the greater good. One might argue that such a worldview contains internal or external inconsistencies, but that is a discussion for another question. Many if not most people would probably consider this person to be acting immorally.
Case 2: Emotion-Go, Logic-No.
This is the standard case of a person having selfish urges but also having enough self-control to prevent acting on these urges by way of a moral code or worldview that says the action would be immoral. Many if not most people would probably consider this person to be reasonable and moral here.
Case 3: Emotion-No, Logic-Go.
Having a harmful and selfish mindset but lacking the emotions to act on it is to be immorally impotent. Aside from spreading the mindset to those who possess the emotions needed for action, this case is rather uneventful. On the one hand, such a person could be called potentially immoral. On the other hand, the deficit causing the lack of actionable emotion could itself be the result of a deeper deficit, which may in fact be the cause of the mindset as well. Hence, removing the core cause could remove both "problems", thus invalidating the hypothesis of potential immorality.
Case 4: Emotion-No, Logic-No.
A person lacking both thought and drive for immoral action is one without moral dissonance. Whether such a person is more or less likely to become immoral with a change of circumstance is highly open-ended and speculative. There are too many factors in practice of the general case. Moreover, a current lack of urge for selfish action is not to say such an urge never existed. Perhaps the person has transcended that old mindset after a long bout of ill ways.
Probably most people would agree that self-control, or the ability to suppress basal urges, is a positive quality in itself, but is this to say that lack of self-control is inherently immoral?
I would argue no, if one lacks selfish urges in the first place, then that agent's existence is inherently safe from being immoral. Obviously an agent or even non-agent can cause harm in other ways, such as through incompetence or other sub-optimal features; but calling an agent immoral from general lack of fitness is to say that only the most currently fit beings in society are moral. Moreover, since fitness is relative, every person below the very single best would be immoral. And since fitness is circumstantial, the most moral beings could easily become immoral with a strategic change of environment.