The Sefer Chasidim writes that when you understand the reason it is not Nichush. In our case we aren’t doing it as a superstition at all. Whoever does it knows or thinks that it is for a reason.
The Chida’s main point is that in order to render an act a Derech Emori you have to be sure it is baseless. The most you can say is that you didn’t it in eearly sefarim and the key looks like a cross.
As to Chukas Hagoyim, the Issur is to follow them and their culture. That is not what we arte doing. If people did it once to follow their custom then they would have been Over the Lav. We don’t know how it went over — granted that it did — and we definitely aren’t following a Goyishe custom.