I think the rules on numbering were much more lax back then. You basically chose your own number didn't you?
1974 was the first full season that drivers ran with the same number in every race. Previously, race organisers had allocated numbers on a race by race basis. For 1974 until 1996 there were fixed numbers for each driver but apart from the previous year's WDC team, the numbers did not relate to past results. Hence, in the years when they weren't entitled to 1 and 2, Ferrari ran their cars with the numbers 27 and 28. From 1996 to date the previous year's WDC team gets 1 and 2 and the other teams get the numbers that reflect their position in the previous season's WCC.
Almost right, they were inteoduced in 1974 based on the finishing positions of the 1973 WCC. Permanent numbers had been introduced mid-1973 previously, I think based on the then standings.
From 1974 on the cars kept the numbers, with the exception of the WDC's team adopting 1 and 2. The team that previously had those numbers swapped, so for example Ferrari had 27 and 28 in 1989, Prost moved there as WDC and took 1 and 2 with him. McLaren swapped to 27 and 28. The following year Senna won and they swapped back. The teams didn't choose them. That's why Tyrrell had 3 and 4 for so many years.