This is a firmware designed for noobs (in my opinion it just makes things worse). So:
1. There is no program to make skins for fw 3.0 and theres no space left on the NAND for it because of all the games they preload.
2. They replaced the utility menu with a "help" menu. Its no big deal though, just rename .gpu files to .gpe and run them with the games menu.
3. Yep, they figured you don't need to access the NAND becuase they filled it with games. It is possible to access it though, you can telnet into the GP2X or use Sterm, but you need basic knowledge of the linux command prompt (remember DOS?, didn't think so).
It is however possible to downgrade to 2.1.1 To do this you must first install 2.0.0, then 2.1.0, then finally 2.1.1. This should be fairly easy to do, all you do is extract the files from the installation package of 2.0.0 and reboot while holding start + select, then repeat for 2.1.0, then finally 2.1.1. But if you do this you should plug your GP2X into a power supply, becuase if the batteries die during a firmware "update" you now have a brick.
Since the bootloader for 3.0 and 2.x are the same, don't put the file named gp2xboot on the SD, this way you have no chance of bricking your GP2X.