1. No, it is probably better to use and keep all data. Repeated-measures methods as implemented in functions like `lmer` from **`R`** package **`lme4`**, and `gam` and `s` from **`mgcv`** do not require higher-level units to have the same number of lower-level units. 2. Yes. Cross-validated accuracy measures are likely less optimistic when you sample at the higher than at the lower level.