Ha ha ha.... you like to make it complex isn't it
stacking both WPAD stuff with reverse proxy while none is fully working yet
WPAD is a bit tricky because you do not control what browser is going to search as web server providing wpad.dat file. e.g. Depending on what you have set, assuming you use DNS method, then browser will try different URL based on host FQDN.
and something like htt:/whatever/wpad/ will never be searched... is will rather be
http://wpad.whatever/wpad.datThus I'm a bit confused with your example
Then if you have index.html file at wpad.home.lan, this is different
(I've one for testing purpose...)
This said, if your goal is to reach
http://wpad.home.lan when you type
http://aaa.dyndns.org/wpad/ then you don't need any rewrite here but redirect to
http://wpad.home.lan/ Removing your "rewrite" directive should work so far... let me think twice about this...