you can use:
mknod /dev/ttyACM4 c 166 4
read 'man mknod' and check out '/usr/src/linux/Documentation/devices.txt' for where I got the numbers from.
(if you do 'less devices.txt' , you can do '/ttyACM <enter>' and it will take you down to the section you need... '/' is for search
I think that /dev/modem is usually linked to /dev/ttyS0
ln ttyS0 modem
I don't know about your other question, sorry.