In science fiction it is common to refer to the star around which the planet you're writing about orbits, as a "sun".
Anyway, since even our sun appears as different sizes depending on where it is in the sky, I don't think it's an unreasonable sentence. Another point is that at some stage a learner has to have the confidence that they know what the sentence actually says even if it's an unexpected statement. You can't keep taking a good guess on the basis of the context forever.
When you say "the only one we have, you are imply that there are others, that other life forms have?
Personally, I don't think we should have these unrealistic indefinite phrases, but the course design thinks we should learn the nouns before the articles, presumably because the article is quite complicated. The reality level does not seem to come into it.
I know it doesn't make sense grammatically, but to me it sounds very much like she's saying "grian a mhor" rather than just "grian mhor". Is this just a quirk of this particular recording, or does that aversion to certain consonants being together that causes the 'extra' vowel in words like Alba and Glaschu also occur across word boundaries sometimes?
This is an easy misunderstanding, as a word can lenite in one situation and not in another, with no obvious difference in the situation. The mistake, which can sometimes result from bad teaching, is to think that it is something to do with the word that causes lenition. It is not. It is to do with the previous word. Therefore, if there is no previous word, there is no lenition. (There are a couple of exceptions, where the previous word has disappeared, or where the lenition is unexplained, but you should only lenite if the previous word causes lenition, except when advised to the contrary.)
In this case it is the feminine singular definite article that would cause the lenition
A' ghrian mhòr
but as there isn't one here, we have
So here the mhòr is lenited, not because it is the feminine form of the adjective, but because it is following grian (which is feminine).
Please say if you think this is clear, as it is always difficult finding an explanation that is correct without be over complicated.