Always happy to be able to help, if you have any questions feel free to ask them here.
Yes, 12 hours is the default range, but this can be changed to 24h (or a different amount, even if we don’t recommend going beyond 24 hours) by clicking on the “gear” icon high on the right, set the new amount in the first box (and edit the second one if you desire the track length to be longer/shorter), and then save the new settings by clicking on the green “Save” button.
The helicorder view will then automatically resize and rescale to your new input settings.
For your second question, DataView does not display (or calculate) magnitudes of events. I would advise, if possible, to check it to your country local seismological institutes/systems, and see if they can provide a mean of comparison.