See the Hardware Notes supplied with each release of FreeBSD for a more complete list.
FreeBSD supports many software modems via add-on software. For example, the comms/ltmdm port adds support for modems based on the very popular Lucent LT chipsets.
You cannot install FreeBSD via a software modem; this software must be installed after the OS is installed.
No, and there is not likely to be.
Broadcom refuses to publically release programming information for their wireless chipsets, most likely because they use software controlled radios. In order to get FCC type acceptance for their parts, they have to ensure that users cannot arbitrarily set things like operating frequencies, modulation parameters and power output. But without knowing how to program the chipsets, it is nearly impossible to write a driver.
There is a list of these in the Serial Communications chapter of the handbook.
Some unnamed clone cards have also been known to work, especially those that claim to be AST compatible.
Check the sio(4) manual page to get more information on configuring such cards.