I also think it would be better if a Frame had been calculated by Zorro internally by the number of minutes, not the number of bars.
Bars can be absent for some reason in the historical data or in RT, but that should not be shifting frames..

Yes, one can code around this situation, but why would not Zorro take care of it by default?

I have no example of special bars that use Start and Time. For not starting a bar, return 4. - It's the same as with most software libraries: What's often used is the default, what's rare or unusual must be explicitely coded.

Ensuring that the Timeframe always covers the intended period is on the side of "proofing" and "making the functionality more robust", rather than a "special request", I believe.

Question:
when TimeFrame=0, what value will priceOpen() report? From which bar?

I guess from the current bar. But prices of a time frame are only defined at the end of the frame.

It turns out, priceOpen will be equal to an open of the previously set TimeFrame bars ago.
E.g. if the usual/previous TF=4 and currently we are skipping bars (TimeFrame=0),
then priceOpen()=Open[4].