Generally words that cause lenition used to end in a vowel. Sometimes they still do, as dà still does, and quite often the word does in related languages. Two and zwei do, and deux and dos would if someone hadn't decided to stick an s or x on the end to make them look plural.
As for aon, Wiktionary cannot find any forms with a vowel and eDIL says it was óen but with a few vowel variants (that seem to be mainly feminine forms). The only thing I can suggest is that any of the forms in Wiktionary that ended with an s might had a feminine version ending with a vowel (like Latin una or French une). So can anybody help?
It's generally dà unless there's a good reason for the h. So we can't really answer here - you need to you ask where dhà appears, and someone should be able to explain.
However, there is one unusual feature of this word - when you are counting, like '1, 2, 3, 4', you say a-dhà.
There's at least two issues here. One is that it is a practical limitation of the system that they cannot include every possible mistake that people might make as a valid answer. In an ideal world they would accept everything that shows you correctly understand the Gaelic.
The other issue is that there are a lot of distinct similarities between the Celtic languages and English, and they often choose the English that matches the Celtic, such as I am singing rather than I sing, and these are precisely the things that catch speakers of other European languages out. So it may be that good advice for you would be to choose the English that is closer to the Gaelic, whenever possible. Also do not go for the 'correct' English that you may have been taught as school. 'Correct' English has often had the Celtic edited out. For example, I bet you were taught to say you and I as a subject. This is not universal. People often say me and you, which is closer to the Gaelic.
There are a few places where French will help. You understand noun gender, but the biggest thing that it will help with is the T/V distinction. Many languages have ways of saying 'you' politely, but Gaelic and Welsh are, as far as I know, the only two that match French exactly. D