A: No. Different hardware (A59sT has Snapdragon 435 vs MT6750). Flashing will brick the device.
A: Yes, for non-China users. Global has Google apps, English by default, no region-locked apps. oppo a59s global firmware
(Settings → About Phone → Model Number) before flashing. If you see “CPH1717” or “CPH1719,” the Global firmware is safe. For “A59sT” or “A59m,” do not proceed. Need further help? Visit the OPPO community forums or XDA Developers thread for OPPO A59s (search “CPH1717 stock ROM”). English by default
A: Yes, unless done by OPPO service center. However, the A59s is old – warranty likely expired. oppo a59s global firmware