| Quote #1
[…] and therefore, whoever could find out a fair, cheap, and easy method of making these children sound and useful members of the commonwealth would deserve so well of the public as to have his statue set up for a preserver of the nation. (2)
Swift is mocking the British approach to public policy, which emphasizes economy first. Absolutely everything has to serve a purpose.
| Quote #2
As to my own part, having turned my thoughts for many years upon this imports subject and maturely weighted the several schemes of other projectors, I have always found them grossly mistaken in their computations. (4)
This is the cue for the narrator to start throwing out wild estimates of the number of people in Ireland.
| Quote #3
The question therefore is how this number shall be reared and provided for, which, as I have already said, under the present state of affairs is utterly impossible by the methods hitherto proposed. (7)
So far, the narrator hasn't actually stated which methods have failed. There's a lot of political posturing going on, which Swift knows is essential for any kind of political tract—satirical or not.