1)
The potential form はいれる is more common, but いれる is also possible. In the latter case, the sentence means "If that were my backyard, I would jam more than ten people in there(but actually, the backyard is not mine, so I won't do that)".
2)
As a concrete part of body, like as asking the part you feel pain at in your example, 首 is the neck and the head is 頭.
3)
That's ほう. 多い方から3つ means the top three in order of largest to smallest (by the number of the patients in this case). Thus, 少ない方から3つ means the top three in order of smallest to largest.
4)
not clear
5)
the latter