Is it odd for you to hear somebody waiting in a country says "have a safe trip back home" to somebody coming back to the country? It sounds good to me, but is this the word when you see off somebody??
Are there such expressions like followings?
have a safe trip going back home. (no COCA hits)
have a safe trip coming back .
Is would not be odd to hear somebody in Japan say "have a safe trip home" to somebody coming back to the country (presumably in writing or by audio means) .
But, why not make it simple? Just say "have a good (nice) trip". Whether coming or going or coming back before you leave (which I think was one of your questions) this one expression takes care of it all. The French do the same thing with 'bon voyage'.