New line, same bar? Please help

Edit → Measure → Split Measure Before Selected Note ?
https://musescore.org/en/handbook/measure-operations#split-join

You can't cross over to the next line/system without creating a new measure, so split measure is indeed the answer. After you did the split you can make the barline invisible (select, hit V) and should set the 2nd measure to get excluded from measure count (right click, measire properties)

You will have problems with all of the above suggestions when it comes to splitting the tuplet. You are going to have to count notes going into a measure before they are entered and split the measure before entering notes. You can then enter a tuplet that will fill the measure.

To add a little more help, If you decide 14 notes can be on a line, you will need to use half the measure (dotted half note duration in 6/4) and turn that into 14th-let. First split the measure into 2 dotted 1/2 rests and split the measure before the second rest. Then you can actually select both of those measures and turn the rest into a tuplet with a ratio of 14 to 12 (or reduced to 7 to 6). Then enter your notes. If you have several meausres with the same number of notes, then you can select the measures and press R to copy them. I'd do that before entering notes. Of course if the cadenza measure is longer, you will simply have to determine how long the measure is and divide it up so each group will end on a line. You of course can then make the unwanted barlines invisible.

If the note division for each measure is such that you cannot enter a note or rest with it in a single note, you will need to do something different, which I won't get into now.