Probably because it is a standard colour. There are 12 in Gaelic, and 11 in English. You use these when giving a general description. Words like teal are not standard colours and are extremely specific. They give a sense of precision that is completely inappropriate for glas that covers the darkish-bluish-greenish-greyish range.
Teal is about 1000 times less common than blue, green and grey. It gained a brief boost peaking in about 2000. I think this was when computers could only handle a limited range of colours and that range expanded to 16 colours where the word was introduced in 1987 and then popularized by Microsoft, including teal as one of its basic 16.
Yes. The original proto-Celtic word glastos covered a range of colors that English speakers would call green, blue, and gray. Over time, in most of the descendant languages, the descendants of this word have narrowed in the range of colors covered, as additional words have been adopted to distinguish finer shades of color. In Welsh and Breton, the meaning of glas has narrowed to primarily blue and in some circumstances green. In Scottish Gaelic, Irish, and Manx (glass), it primarily means gray or green. In Cornish, it can mean blue, green, or gray.
In Beag air Bheag on LearnGaelic, I learnt 'liath' for grey or blue, gorm for blue (or green in nature - as per the notes in this course). For example, in BaB we're taught 'falt liath' (grey hair). Can you also say 'falt glas' or would you not use 'glas' for hair (a bit like 'dearg' vs 'ruadh')?
Yes I think you may be confused. This sentence has agam from aig in it so it is a 'have' meaning not a 'wearing' meaning.
Note that, contrary to what you may have read, aig is not strictly 'own'. You can use it for your wife, your rented car, your teacher and lots of other things you don't actually own. Another preposition, le is used when you want to be explicit that you 'own' something.
Yes. Most people only get the opportunity to learn Germanic or Romance languages at school as they are more prestigious than the Celtic ones. So there are a lot of new features here, most of which are shared with the other Celtic languages.
But Scots and English also have a lot of similarities to the Celtic languages that other languages don't. You may not notice these as differences are more noticeable than similarities, but the Celtic languages are much easier if you speak Scots or English.
Scots has a few extra similarities to Gaelic, so keep a look out.