Learn English – Difference between “the other” and “another”

You just keep quiet and be good.

Keep and be are imperatives.A noun or a pronoun can be used with imperatives to make it clear who we are speaking to.

Yes, use rather than.

These two examples do not seem correct to this native UK English speaker.

He wanted to be an actor over a comedian.

Can we come over on Saturday over Friday?

This usage of over seems acceptable:

I would choose Brand X over Brand Y

The difference being that we are explicitly using the word choose

In my own work I would use

I would choose Brand X rather than Brand Y

I would choose Brand X in preference to Brand Y

Generally, I would use rather than or in preference to instead of over (or indeed use instead of). This may be a matter of personal preference, but for me the word over has so many meanings that the chance of ambiguity or clumsiness due to repetition is too great.

For example look at the two uses of over in the same sentence in your example:

Can we come over on Saturday over Friday?