I followed the tutorials http://wiki.maemo.org/N900_USB_router / http://wiki.maemo.org/N900_USB_networking and got the network itself running. The problem is the "USB router" part which works only in a quite complicated way.

I did it in the following order...

1. Start up "Mad developer" (Installed drivers on Win7) , select "USB networking" and "Windows network" afterwards. Without this step my NB is not detecting the USB network device at all. Afterwards I have always a working USB network connection and can connect for example via SSH to the phone.

2. Change the IP from 192.168.2.15 to 192.168.3.15 in /etc/network/interfaces else the last command of step 3 will defiantly fail even if I change the number in that cmd to .2!

3. Then I disconnect the device, use "ifdown usb0" and "ifconfig usb0 down" ... and write the followings cmds:
It executes in general all cmds fine but the last one, there I get often the annoying error "dnsmasq: failed to bind listening socket for 192.168.3.15: Address already in use" and need to start all over again.

4. Enjoy stable / fast internet and no battery issues, in general way faster and more stable than Joikuspot / Mobilehotspot.

Are there any easier ways to share the WLAN/3G connection of the phone via USB for Win7? How can I avoid the "dnsmasq" issue or prevent that "Mad developer" will block the IP for "dnsmasq" once I set it to "Windows network"?

Nobody has an idea how to solve my issues?

Have patience. Someone who knows the subject will likely read your question in the near future.

Darn, low on answers. You could try making a new thread with the subject title line starting with "[Help request]".