I haven't done any quantitative studies on it, but I imagine such a situation would indeed be quite rare. It depends, somewhat, on the phonological context of your language. Is hiatus avoidance something that there are synchronic phonological rules for? As in, do underlying sequences of vowels occur, which are then resolved somehow, or do they just happen to not occur for whatever reason? It could be, for instance, that your language lost some consonant specifically between two <a>, such as /h/ or /ŋ/.

That depends on what you mean by "double 'a' vowel". The way you transcribe it, it looks like you have a syllable boundary there, which means that unless you have an intervening glottal stop you'd be violating your own language's hiatus rule. If you mean something else, I highly recommend you take a look at this convenient transcription method to make it more clear.

Yes, this is reasonably common. Luganda, for instance, does not allow two vowels in sequence; where such sequences would occur, it lengthens the second vowel and either deletes the first or converts it into a semivowel if the first vowel is high.

For a more direct correspondence to your strategy, some dialects of English insert /r/ between consecutive vowels.