You should use the variable MIGPUMA1 (PUMA of residence 1 year ago); MIGPUMA (PUMA of residence 5 years ago) is only available for 1990 and 2000 samples. You should also combine it with MIGPLAC1, which reports the state that a respondent who had moved in the previous year moved from. You are correct that respondents with MIGPUMA1 = 05700, MIGPLAC1 = 048, AND (PUMA != 5700 OR STATEFIP != 048) are those who lived in Guadalupe county, Texas one year before the survey, but no longer did so at the time of the survey. A different way you might define these respondents are those with MIGPUMA1 = 05700, MIGPLAC1 = 048, AND MIGRATE1D != 23. By excluding respondents who moved within the MIGPUMA, you are only left with respondents who moved homes by leaving Guadalupe county/MIGPUMA. The ACS is fielded throughout the year, so the respondents may have migrated out of the county/MIGPUMA in 2020 or in 2021. You may be also interested in the recent release of the 2022 ACS 1-year file where Guadalupe county is contiguous with PUMA and MIGPUMA 05700
The 2021 5-year data append each of the five 1-year data files (2017-2021) into a single file. Your estimates using this file will therefore include all respondents who reported migrating within this 5-year period. The 5-year ACS file divides PERWT and HHWT from the 1-year files by five in order to make sums more easily interpretable as annual estimates during the five-year period. If you were to sum PERWT in the 5-year file for respondents who you identified as having moved out of Guadalupe county, your estimate will be the average annual out-migration from this county over this period.
I’m not sure what Population Estimates Program you are referring to. However, the county-to-county migration flow data released by Census are in fact derived from ACS data. However, estimates using the Public Use Microdata Sample (PUMS) file are not expected to perfectly replicate official estimates because the PUMS data are subject to additional sampling error and further data processing operations (see the 2022 ACS PUMS Accuracy of the Data report).