Yes, it's 256MiB.
FlashGBX officially supports the 369-in-1 bootleg cart (6600M0U0BE), read/write wouldn't be a problem.
You can.
The 369-in-1 bootleg cart has a 64KB SRAM chip, so SRAM works out-of-the-box, FLASH512K and EEPROM need SRAM patching, and FLASH1M is a no-no since you need 128KB...