(This was written before @Choko posted her comment but the two seems to agree.)
The easiest answer to your question is no, it is not that simple. It depends on the nature of the verb and what you want to say.
You reference to "present continuous" suggests that your are not familiar with concepts such as "stative", "durative", "punctual", "subject change" and "continuous change" verb groups or the resultative state in Japanese.
This is quite a big subject to explain (there are probably several theories) but it is the key to the solution you are looking for: I found this paper, from snailboat, quite useful: http://web.archive.org/web/20161103070805/http://homepage3.nifty.com/park/aspect.htm (via Web Archive, since the original link is dead)
BRIEF EXPLANATION OF UNDERLYING GRAMMAR & USE OF ADVERBS
There are not really any short cuts to studying something like this but, in relation to your question, I would summarise this as follows:
Verbs like 来る can be classified as "subject change". In its plain form 来る describes an unexecuted and (probably) future action. In its ている form, 来ている, it usually describes an action that has been executed and the result is still being felt (eg the subject has come and is still here). This is also referred to as in the resultative state. These verbs are different from verbs such a 騒ぐ which in its ている form describes the continuous action (or state) state of making noise rather than a "resultative state".
As you know, adverbs such as まだ and もう are used to describe or refine the action. This can be can be quite important. For example, in the case of
来ていない, as the negative form of 来ている, describes the (opposite) state that has not happend. The subject has not come (and is therefore not here). The adverb まだ reinforces this meaning and might be used to imply that somebody was waiting for it to happen or it was over due.
However, getting back to your question, the adverbs もう and まだ can also be used with such plain form verbs in the negative form but their appropriateness also depends what you want to say. For example:
He is not going again. (ie in the future)
Further, although the ている form of subject change verbs are usually used to describe a resultant state this is not always the case. One way to describe a continuous state using a subject change verb is by choosing an appropriate adverb. A good example is the verb 死ぬ, to die. The following sentences should illustrate this:
僕の猫は死ぬ = My cat is going to die.
僕の猫は死んだ = My cat died.(see note 1)
僕の猫は死んでいる = My cat is dead.
僕の猫はだんだん死んでいる = My cat is slowly dying.
By adding the adverb だんだん the predicate 死んでいる has changed from resultant state to continuous state.
So perhaps the answer to your question is:
No. It depends on the nature of the verb and what you want to say. If the combination of an adverb and a verb does not feel right the best course of action is to consider what you want to say and whether the sentence you have made really means what you want it to mean. This is illustrated by the other example you give:
(A) is correct if you want to say: No I have not bought it yet. (B) might be correct if for some reason you wanted to say "No, I am still not going to buy it." (note 2)
1. I am not sure if this really flies as an explanation of the difference between 死んだ and 死んでいる so I have made it a note:
If we accept the saying a cat has nine lives then perhaps we can say it has to die nine times before we can say it is "permanently dead", as in it leaves this world for next. If we can say this then perhaps before it loses its ninth life we can also say:
僕の猫は死んだことが8度ある：My has cat died eight times.
...and it would not be correct to use 死んでいる because the cat is still enjoying its final life.
2.This is consistent with @Choko's comment who suggested that you might not be buying because you were expecting a discount to be announced.