I’ll address each of your questions one at a time.
(1) Yes, CPSIDP uniquely identifies individuals across CPS samples. This variable allows linking of respondents, based on the 4-8-4 rotation pattern. CPSIDP will only ever appear a maximum of 8 times, which is the number of times a household may be observed in the CPS survey. In some cases, individuals will appear fewer than 8 times due to migration, mortality, non-response, and recording errors. Extensive documentation about the creation of CPSIDP is available here.
(2) As you point out here, there are several time intervals where linking individuals across samples is not possible. As is noted on page 126 of this paper:
Data from 1962 to 1978 present the most serious linkage challenges. Each housing unit was assigned a unique identifier during most (but not all) years in this period, but person-level identifiers do not reliably identify the same individual in multiple samples. […] Furthermore, because of changes in the numbering scheme for housing units, household-level identifiers cannot be used to link housing units between 1962 and 1963, 1971 and 1972, 1972 and 1973, and 1976 and 1977.