That's how I understand it too "Two" is special - it is called the "dual", and once upon a time lots of languages had it, including those that evolved from proto-Indo European, such as Sanskrit or Old Greek e.g. and their descendants. Most of these have lost this form a long time ago, though you find "leftovers". In English. "both" as opposed to "all" for instance. Gaelic, Irish and a number of others like Slovenian kept it
The idea that two of a kind should be marked grammatically is also found in other language families, such as modern Arabic and Hebrew, though the latter IIRC only for certain objects such as body parts that come in twos, like "ears".
That is exactly correct. Old Irish has the distinct dual, just like the other old languages you mention. Over time people forgot the special endings and just used the singular. You mention some languages that have small remnants. Old English retained special words for 'we two' and 'you two', while losing other features (apart, of course, from the b- words like both that you mention).
I am intrigued with what you say about Arabic and Hebrew, as there was exactly the same remnant in Middle Welsh. I think it has virtually disappeared, but I have not finished the Welsh course yet so I'm not sure.