It does have 512Mbit of NOR but nothing uses it other than a couple of beta loaders for the EZ5. Making a multiple GBA ROM loader is not the easiest and with the 3 in 1 you also have the save issue -- if you are going to fill up the NOR then you also need to hold all the saves at the same time. The 2Mbit I think it was that the 3 in 1 uses is fine for any single GBA game, put two copies of a large flash save (like pokemon) on there
There is no real time clock for pokemon to use with the 3 in 1 -- the GBA did not have one in hardware so it had to be added at cart level. Some flash carts (including the EZ2 and EZ3) added a real time clock chip to their carts, however PCB space, cost, increased battery drain.
I am not sure why clock battery is used, I guess a lot of people call those type of batteries "watch batteries" and that might be a mistranslation at some level.
With no clock to speak to the game gets upset. However people patched it so the clock advances when the game is played (some went faster but most you will find today do not), about the only thing this really troubles is berries for normal berries you wait 24 real hours but 24 hours of actual gameplay is a bit different.
Yes you can trade with another GBA game, if you apply too many patches it can trouble the link code but it is usually just a matter of removing the patches. Theoretically it should all work fine but if you can test it before you turn up somewhere where you are going to struggle to fiddle with things then do so. If you are going to want to use pal park on a DS then you will need to patch the DS ROM (or dump your DS cart's save, run it with a patched ROM and export the save back).
Can you use a DS to write to the EZ4 without a PC.... technically yes but don't do it the main tools out there have a nasty habit of bricking the cart. I have no idea what goes with the new microSD ez4 as I am not inclined to test mine. There is nothing special about EZ4client though - if 32 gigs of microSD is not enough for you (and as GBA games are usually 8 to 16 megabytes then that would be an impressive collection) then bring a USB drive with patched games already on it and copy them (and a save) across.
Nope, all GBA carts that are not a supercard, supercard clone or some of the very first GBA flash carts will run everything as far or slow as it would have ran on the original cart. Multiplayer should be fine, about the only real difference for anything is as they use SRAM to save (and patch the games to use SRAM rather than the EEPROM, Flash, or other type of SRAM they originally used) then you are going to struggle to link up with a gamecube for things like pokemon stadium and the sonic chao adventure thing -- much like pokemon pal park for the DS games then you will need to alter the GC games and nobody has bothered to do that.
10 was already taken (it is DS program), if instead you meant the DS ROM dumper (wood dumping tools or rudolphs) then originally you could but the SDHC one and modern loaders lack a DS mode to use those with. There is no easy way to dump GBA games using a GBA slot flash cart.
11) Yeah the very first ones are 256Mbit of NOR, 128 of PSRAM. When the 256 NOR chip dried up the EZTeam moved to 512 but nobody uses the extra 256 save for that old beta loader on the EZ5 which I already mentioned.
12)
3 in 1
Cheaper
Possibly easier to use thanks to being managed from a DS cart, not the the EZ4 is hard to use of course.
Can easily fake a game for the purposes of the DS GBA slot reads (things like have a GBA advance wars in and get a bonus on DS advance wars), save for pokemon pal park which needs you to patch the DS ROM
http://filetrip.net/nds-downloads/rom-hacks/download-pokepatch-4-2-f27240.html
EZ4
Standalone flash cart. The 3 in 1 really needs a DS to work well, you can write a single ROM to the NOR and have it boot in a GBA but it is not my idea of a good time.
External memory is cheap and easy to replace.
Cons.
The 3 in 1 has a less than great battery which you might have to replace.
The EZ4 is more expensive. You can not easily fake a GBA cart for DS thing I mentioned above, though PAL park should work.
Second post.
Older games used SRAM, newer ones technically use something like it which does not need a battery.
Anything that has a DLDI patch can use rudolph's exp loader to manage the 3 in 1 -- the EZTeam released the source code/access libraries to the cart so that could happen.
GBA games need fast memory to run (it is why the supercard GBA slot stuff is not great), NAND memory (like that in SD cards) is not fast enough to run GBA games directly from it. The two main types of fast enough memory in the GBA era were PSRAM and NOR.
GBA games went up to 256Mbit, the EZ4 main model only had 128Mbit of PSRAM so it also has NOR for those few games above 128Mbit. You can also use the NOR to save having to load games into the PSRAM. You manage all this on the EZ4 itself.
NOR memory will hold the data until you delete it.
I think we covered the battery stuff earlier. That said button cell = watch battery = coin cell = סוללת כפתור if my dictionary is correct. It is used to hold saves while the GBA flash cart is not in a powered GBA, when you next power the EZ4 it is written back to the microSD card, whatever you manage a 3 in 1 with should do the same or provide you the option to do it.
Anyway I am repeating myself and what migles said.