Raspberry Pi and USB modem - can I connect it like this?
The RPi, regardless of version, takes 5V in and does not regulate it past the input. The USB ports' VUSB pins are connected to the 5V input. Depending on the RPi revision, there are one or two (in everybody's opinion) undersized fuses in the way, as well as (in everybody's opinion) undersized power and usb capacitors. By doing what you have described, you simply bypass the undersized fuses. Same as if you had soldered a jumper across the fuses. This is pretty common mod for the RPi. So you should be fine, it will work as you believe.