Thanks, already expected that you probably know the most about this stuff 
Just to make sure: I cannot use a copy of an old household registration, can I? Since my mother does not have an ROC passport anymore, can she also reactivate her registration after entering with a foreign passport? ("…或入國証明文件入境始得辦理遷入(恢復戶籍)")
My Chinese is unfortunately not good enough to fully understand all legal texts.