After a lot more testing I come to the conclusion that simply my old observations must have been wrong (maybe I shoudn't do setups at late night) ;) .
Summarizing this meas: the confusion started with the fact that the bridged LAN IP can be accessed from the outside network (ping and webinterface and ssh). All other observation where just due to other effects, e.g. the pptp stuff still works.
Hence, what remains from here is just, that it's probably a bug that the internal ip can be accessed from outside in bridge mode. Furthermore, this cannot be deactivated by deactivating the anti-lockout rule. This is discussed here: http://forum.pfsense.org/index.php/topic,5441.msg32479.html#msg32479 .
Sorry for the long post and the confusion.