In your example you are really just saying that if she didn't fancy the ice cream you are sorry about it. It's not clear whether you are implying that you know she didn't or you just have no idea. This depends on the context and I wouldn't regard it as grammar.
The other case you mentioned was different:
Scusa se non mi sono fatta più sentire.
The reason why in this case the woman who says this sentence knows what happened is because she did it. It's not a general meaning of se, it's just an extension of sentences like these:
Scusa se...
Mi dispiace se...
They are used this way so often that the original meaning of se got lost in the formulaic expression, and they are commonly used as if they were the same as
Scusa che...
Mi dispiace che...
You don't use se that way in many sentences that do not mean apologising.
You might use it in sentences that mean not apologising, like
Non mi dispiace se non mi sono fatta più sentire.
Me ne frego se non mi sono fatta più sentire.
but this is again the effect of the formulaic expressions about apologising, because you would not say
Sono orgogliosa se non mi sono fatta più sentire.
You would say
Sono orgogliosa che non mi sono fatta più sentire.
if you are a woman who's proud of resisting the urge of calling her ex, for example.
It's not about se, it's about scusa and its siblings.
The future sentence should be
Scusa se non ti piacerà il regalo.
and it sounds OK. It is ambiguous whether who says that already knows that the gift is not going to be liked or he genuinely doubt it or even he is just behaving modestly. Context can tell.
Scusa se non ti è piaciuto il regalo.
doesn't sound bad at all. It's absolutely fine and it could imply anything about the giver's expectations.
The last sentence should be
Scusa se non ti fosse piaciuto il regalo.
In this case it definitely means that the giver has no idea whether the receiver liked the gift or not. The subjunctive mood is used here to convey a sense of hypothesis in a stronger way. This form is used for impossible, unlikely or unknown conditions. It sounds a bit weird because tenses and moods are mixed up in an unusual fashion, but it's not wrong. I'd rather reshuffle it like this:
Se non ti fosse piaciuto il regalo, scusa.
In general, as @DaG noticed above, if you change the order by separating scusa from se, the ambiguity disappears and it becomes a proper causal if. This reinforces my conviction that scusa se is just a formulaic expression.
