Here's my thoughts on the individual items you mentioned:
Luxembourg - I am actively considering a change here.
Ireland fascist - This is not an original design. This is the flag of the Irish National Party.
Germany monarchy - Yes, the black/white/red tricolor is the authentic German monarchy flag. But I play Germany frequently and I want a flag with a more monarchist feel. I realize looking at this, though, that I can use the authentic war ensign (http://commons.wikimedia.org/wiki/Fi..._1871-1892.svg) now that I'm not worried about centering.
Manchukuo/China republic - This one is complicated. I think that the use of the five nations flag by the Japanese puppet state makes the flag feel wrong as a generic republic flag for China (even though its origin predates the occupation). I guess you could make the same case for Manchukuo, but that state's independence was a Japanese artifact.