About your question NO.1 , I have done some testing recently. I am using the new EZ Flash IV MicroSD version on my DS lite.
After using the EZ4_Client to write the gba roms into the TF card, I choose to write two games into NOR (the NOR need to be formatted firstly).--- The two gba roms are "Densetsu no Stafy" and the "Advance Wars 2 Black Hole Rising", and these two games add up to about 23-24MB. I play the two games crossingly--- first play one and then play the other so see if they save properly.
I have tried with different methods. Firstly, playing the Advance Wars 2 first, and when play to some stage, I choose to save it. Then I turned off my console, and then power on and play the Densetsu no Stafy, and in the middle I press START button to save this game. Later on I play the Advance Wars 2 again and find that my previous game saving still exists and can continue to play. Again I power off the console and then power on to play Densetsu no Stafy, which also can be continued to play.
Here is my conclusion/ suggestion:
IN ALL, because the different methods of saving with different games(such as Densetsu no Stafy saves by press the START button and power off, while Advance Wars 2 saves by press A button during the game and then choose to save following the hint ), every time you are about to stop playing you should choose to save the gamestatus before you play the other game. Once you forget to save the game, I have to say it's a big tragedy for some games (For example, Densetsu no Stafy which saves by press the START button ) as you need to play from the very beginning again. While for other games like Advance Wars 2 (save method referred to above) , you may get a little lucky since its first save still exists and you can continue to play.