1) There is a GBA and DS menu, as far as I know the m3 real nopass works with it.
2) Yes, make sure to get a good one though if you want to run DS games.
3) Before DS slot cards appeared people used GBA slot (I still do). It can play pretty much all GBA and DS games and homebrew (pre DLDI DS stuff is a bit sketchy but everything that matters has been remade or updated).
4) There are lite sized versions and a GBA cart sized version. The GBA cart version uses miniSD (you can get an adapter for microSD though) and the others use microSD. I am in a bit of a rush so instead of running down the differences I will have to take the lazy option and say look at the wiki (either here or
http://ezflash.sosuke.com/ ) or one of the many posts on the subject.
For what it is worth I have been using a lite sized cart in my original DS and GBA for about 2 years now) at one time in a custom case and when I broke that in a slightly modded lite case. You can also tweak a GBA case to use it (I have seen some of the shovelware stuff go for about $2 and I am sure you could trade some beer or something for a bad game from someone else).
5)Probably not but you can run it on the EZ4 and you can also use a file browser able to read both slots to move stuff:
http://forum.gbadev.org/viewtopic.php?t=14630
6) To each their own, the EZ4 is more expensive but it can run by itself with a lot of memory.
7) Tough one, if you use the built in nopass chances are you will get the DS menu of the EZ4. If you boot to the DS menu you will get the GBA menu.
Bonus points: Rudolph made a 2 in 1 tool so if you want you can store GBA code on the m3 real and load it to the EZ4 ram:
http://www009.upp.so-net.ne.jp/rudolph/nds/EZ4/
Quite why you might want to do this I do not know but you can.
