Thanks! Yes on both:
1. Every cell in stat_icons_16px_sheet.json has a "name" key in the form <stat>_<style>, e.g. "attack_a", "crit_chance_b", using the same stat names as the list on the page and in README.txt. Each entry also has x / y / w / h, so you can look an icon up by name and slice it straight from the sheet. The 32px sheet uses the identical layout, so just multiply the coordinates by 2.
2. The 2x and 4x copies are plain nearest-neighbour integer upscales of the 16px source - no redraw or smoothing, so they match the 16px icons pixel for pixel.