Using either the SCALE or MOTION effects, there are sliders to align and size. And the 'mask' can be dragged in the preview window. Effects can be saved ... click the folder icon
while working on a specific effect. Give it a name. It will appear as just another icon choice at the bottom of the main effects window.
It's unclear what the split problems were. One way to define a clip that's in the bin is to select it and SET START and SET END POINTS (under the preview window). Then click the ADD button and the clip will go to the timeline. Click the chevron to the right of the button for positioning choices. Of course, the clip can always be moved on the timeline.
If a bin clip is to have several cuts taken from it, right-click the clip and COPY <ctrl-C>. Then, in a clear area of the bin, press <ctrl-V> to PASTE - as many times as needed. Each clip will have an incremental #. Trim each copy and ADD to the timeline, leaving the original (master) as is.
For protection, Save Project File AS routinely, giving it a unique # or name, so as not to overwrite prior saves. Using Save Project Files (no AS) will overwrite. From the HOME tab, that choice is available by clicking the chevron to the right of the Save Project button on the toolbar. If you make an editing mistake, press <ctrl-Z> to backtrack, step-by-step.
Once your segment, like the one submitted here, is complete, export it. It's now a composited clip. Then you can work anew on more of the project and add that clip to it. A decent approach is to export each song, individually, then import and arrange them for a final export. That also eases the load on the PC, which can speed things up.