1. I guess most of the "drivers" you use for devices are kernel modules, that are installed into the kernel (possibly at run time with modprobe). they can be added via
modprobe <modulename>
-OR-
insmod <modulename>
and removed with
rmmod <modulename>
to/from a running kernel. the other chance is that they are built in the kernel, so they're there all the time and this is useless.
2. you basically can't

there are some rules, but they don't always be like that...like harddsik partitions are represented by the name hdaX where X is the number of the partition; hda1, hda2, hda3 and so on. tty's are one type of devices, and so on..the numbers after the names depend on how many devices of the same kind are plugged in and so on..but this thing is, too, changing all the time (devfs changing to udev or what was it called and that way...)
3. the /dev/modem represents your modem device. ppp0 means a point-to-point-protocol (ppp) link number 0 (zero), the first one you have opened. it's the way the connection is made, whereas /dev/modem is the device file telling about your modem (hardware)