What’s the best application to create our own custom watch faces?
There is no “app”, you have to code the watchface in Android SDK.
Different way as on W806/W812/W815
Is there a way to convert a watchface .zip file to apk?
No, you can use parts of the watchface from Android Wear (e.g. Pointer, Watchface). But you have to compile a APK out of it.
I don’t know if the Q1 supports watchfaces as the D5 does. And you can’t convert APK (only working on KW88) to Zip. Either you have to code a watchface as APK or by clockmaker (ZIP). You can however simply unzip the APK und use the included PNG.