That is a really interesting question that is not easy to give a full answer to. 2-3000 years ago, it appears that many Indo-European languages had three full sets of endings for nouns, adjectives, pronouns and verbs, one each for singular, dual and plural. It can be assumed that in those days, 'brothers' meant '3 or more brothers', in the same way that it now means '2 or more brothers'. However, over time this dual has become less popular, with various different remnants in different languages. The situation in Gaelic and Irish is that they have forgotten the dual endings, replacing them with singular endings, and use this simplified dual only after the word dà. That means that bràithrean means '2 or more brothers', just as in other languages.
A further problem is that it may not be clear how many brothers someone has, so it is easy to see that any distinction there might have been would easily disappear. But eyes and hands are different. They use of the dual for things that you usually have two of survived into Middle Welsh, so you would not have said ei llawau (a làmhan) 'her hands' but ei dwylaw (a (dà)làmh). So that is at least one situation where you could not use the plural for two of something.
(For anyone who is interested, note that ll lenites to l, so, at least in this example, Welsh is showing identical mutation rules to Gaelic.)
There is a further complication in your example, in that you cannot use ordinary numbers for people in good Gaelic, however colloquial you are being. Notwithstanding the fact that you may have seen examples where it is done on this course, you should avoid counting people until you have learnt how to. D
I imagine that this simply means that you should say for example "còignear reubalaich" (meaning "five rebels") rather than "coig reubalaich" (meaningless), and "triùir bhan" ("three women") rather than "Trì mnathan" - numbers bigger than 1 should usually be nouns and the things being counted should be in the genitive plural case, not the nominative plural (except when that's the same as the genitive plural, as for example it is in "reubalaich" but not in "mnathan").
Yes, mostly, but note that the genitive plural of reubalach is reubalach. When nouns slenderize in the plural they deslenderize in the genitive plural.
The notes say
Plurals - Pattern 2
Many one syllable nouns and ones that end in ach, an and al have a genitive plural form that is identical to their standard singular form.
sgioba nam balach - the boy's team
I think this is making something really simple quite complicated, but it is basically true. I also note the typo boy's which should of course be boys'.