Yes, we can use "could" to talk about possibility in the present or future. For example, don't eat this bread; it could be stale. He was sent to prison two months ago, and could be released next week.
must be/must have is used to show that we are sure that something is true and we have reasons for our belief.
He looks happy, he must be hearing good news
Note that be requires an adjective or noun to follow it, not a verb, so we use a kind of adjective called an active participle hearing. Together with be, this forms a present continuous.
He looks happy, he must have heard good news
Have must be followed by a past participle or by been and a present participle. In this case, the past participle is added, and have heard makes present perfect simple.
In these examples, we state that the reason for our belief is that he looks happy now, but we would need additional information to decide which example is appropriate. The first would be appropriate is he didn't look happy a minute ago but now somebody is talking to him. The second would be appropriate if nobody is talking to him right now but, for example, he has just come out of the boss's office after his salary review.
We cannot directly build a future for this example, as he does not yet know that he will receive good news and so he probably does not look happy, and so we cannot base our belief on the way he looks.
Here is a different example where we can infer a future event from current information:
The blackbirds are singing: it must be going to rain.
Best Answer
English can indeed get a little confusing when you start looking at all the different time tenses you can imply in a sentence, depending on the grammar you employ. I will use your example (future tense) sentence and re-write it to show differing tenses:
The use of 'could', 'would', or 'will be' all imply future tense. The past tense version would be:
In this version, adding "have", "had" or "was" to 'could', or 'should' shifts the sentence into past tense. The present tense version of the sentence would be:
"Can" may be used either as future or present tense, but using "is" or "am" almost always implies present tense.
One of the confusing things about these sentences is that her use of the words "I am convinced" actually sets that specific part of all of these example sentences into present tense; in other words she is convinced (in the present) that in the (past, present, or future, depending on the sentence version used) she (was, is, or will be) unhappy with him, and he with her.