I understand lenition - mostly, now. And slenderizing is starting to make sense. But is there a third thing going on here or am I just not fully understanding one of those?
I don't see why the 'a' in "caraidean" picks up an accent mark when it changes to "a chàirdean". I'm also not sure why all the letters between 'r...d' have disappeared. Or is this just an alternate form of the word?
This is not a regular thing that you have to learn about. It just happens that this plural word caraidean has an unusual contraction càirdean as an alternative form that is commonly used.
So you are just expected to learn it as a one-off. However if you would like a historical explanation, and a reminder of what the accent is for, here goes. When a sound is lost from a word, a nearby vowel often gets longer to compensate. (Linguists use the imaginative term 'compensatory lengthening' for this process.) Here the second a has been lost, and so the first a has got longer. That is what the accent is for. It tells you the a is long. Because the r is now next to the slender d it also becomes slender, which is why the i is now before the r.
This is totally ambiguous. Given that it's simply one word you don't know whether they are speaking to their friends or commenting about Friends. They could be commenting to someone else in a conversation about associations: 'Friends!' in exasperation. So, this should not be incorrect if translated simply as chàirdean.
There are two issues here.
The first is that chàirdean is wrong either way. If talking to friends it would be
but if talking about friends in exasperation it would be
with no h. The lenition only occurs with some good reason, of which the vocative particle a is one.
The second is that there is a convention on this course (and I don't know how you could be expected to know this, unless it says in the notes?) that the exclamation mark is used to mark vocatives.