How is the hidden interface bridge100 working?
(This is isn't a correct answer, but just a starting point.)

Not mandatory

This bridge interface is not at all necessary. Here is the way to inactivate it:


Stop Internet Sharing through the GUI.
Wait one full minute for all the processes to terminate their house keeping:

ps ax | egrep '[ /](PID|Int)'

modify the Internet Sharing plist:

cd /System/Library/LaunchDaemons
/usr/bin/sudo -s
# make a quick backup
cp com.apple.NetworkSharing.plist com.apple.NetworkSharing.plist~orig
vi com.apple.NetworkSharing.plist


within the ProgramArguments array, insert the 2 new arguments -B (disable bridge interface) and -v (verbose output), so that this array definition will look like:

&lt;array&gt;
    &lt;string&gt;/usr/libexec/InternetSharing&lt;/string&gt;
    &lt;string&gt;-B&lt;/string&gt;
    &lt;string&gt;-v&lt;/string&gt;
&lt;/array&gt;


save this new file and exit vi
start Internet Sharing through the GUI,
do a verification ifconfig -a.


Now Internet Sharing is directly working on en1 and all necessary processes are started correctly: pfctl, wifid, natpmpd, bootpd.