Which is the Num key on the keyboard

A small utility, called keyxpat, could help you. You just type the E key and keep it pressed until a metronome gives you some beeps (a few milliseconds). Depending when you release it, you get your é, è or ê. Same for all diacritics.

Disclaimer: I'm the author and I use it in the same situation. I'm a french expat in Montreal with a qwerty keyboard.

Yes, the keys will work the same, provided the physical layout (i.e. number and placement of keys) is the same between your german QWERTZ and english/greek QWERTY keyboard.

For my localized danish keyboard, there is the same number and placement of keys, so I can use a US keyboard layout with no problems on my physical DK (danish) keyboard.