Force piano roll to reflect scale and enter chords/arpeggios

Depending on the specifics of what you are trying to accomplish (which isn’t all that clear) Cubase might not be the best tool for you to use, and a different DAW is your best choice - they all have different strengths & weaknesses. You might also find that the addition of a couple of 3rd-party plug-ins provides the capabilities you need, or that an alternative approach works better than you anticipate.