1. I find it easier just to accept that some names will appear in several forms: Dan/Don, Joney/Judith, Finlo/Phinlo, etc. even when refering to the same person.
2. Unless it is your own work, you can only be sure by going back to the original.
3. For the period in question, the original doucument is itself usually a translation from Manx to English, plus the problems of phonetic spellings.