The screenshot corner-swipe gesture is available when Multitasking is configured for Full Screen Apps - but will not be available (and will be ghosted in settings) when using Windowed Apps or Stage Manager modes:

When using the Windowed Apps and Stage Manager modes, the finger-swipe screenshot trigger gesture is disabled - as this would otherwise conflict with resizing of windows from the bottom corners of the screen.
While the finger-swipe gesture is not available in these modes is not available, the equivalent Pencil gesture is still available if configured in settings:
Settings > Apple Pencil > [Pencil Gestures] >
- Bottom Left Corner - select as required
- Bottom Right Corner - select as required

Your other issues are not understood - and will require more explanation as to your difficulty.