I think your last option seems right - parents could choose which prn to use. It's like, in the US
Caitlin is almost always said like
Katelynn, not
Cathleen. Sure it's the same name as
Cathleen, but most US people probably don't know and don't care. They read and like
Caitlin as "Kate-lynn" and aren't necessarily exposed to information that it originally sounded more as
Cathleen, so they use the
Katelynn pronunciation. So that's a real usage, and therefore it's a legit pronunciation.
However, I have never personally met an Aisl*n* who pronounced it as Ayz- or
Ace-. I've known one
Aisling who was "Ash-ling." I dunno if the ayz- or ace- pronunciation is actually in use, or not. I just expect that it might be. So I'd pronounce Aislynn as
Ashlynn, and if I were corrected I would not argue.
- mirfak