1) There are two modes to run GBA roms with the EZF3in1.
PSRAM, which loads the game temporarily into the EZF's RAM (where it remains only until you turn your DS off). Save files are saved automatically by file name, so you can switch between ROMs in this mode as much as you want without any issues. This is considerably faster than the other mode, NORflash.
When using NORflash mode, the game is written to the ROM portion of the cart. The two benefits of this are that is supports the few games that are 32mb (whereas PSRAM mode only supports up to 16mb), and that the game remains on the EZF until a new game is written over it. These saves must be backed up manually.
2) I'd say between 2-3 weeks.
3) The R4's MicroSD.