Viewport captures the 3D view as it is shaded right now. Current Slot captures the render held in the active render slot. All Slots walks through every render slot and captures the filled ones, skipping the empty ones.

Yes. MSnapshot first draws the view into an offscreen buffer at render resolution, with overlays and gizmos switched off for the shot.

Either you are in the Image Editor, where there is no 3D view to capture, or the fullscreen viewer is running and covering the viewport. Quit the viewer first; hovering the button tells you which case you are in.

That is the viewport being captured faithfully in Solid, Material Preview or Wireframe shading. Turn Show Overlays off in the viewport header before capturing. In Rendered shading MSnapshot already removes them for you.

At your scene's render resolution, including the percentage scale. Every capture therefore comes out the same size, which is what keeps an A/B comparison aligned pixel for pixel.

As PNG files in a snapshots folder created next to your .blend, named snap_... for viewport captures and render_... for render slots. Until the .blend has been saved once they go to Blender's temporary folder instead, so save your file first if you want to keep them.

Open View Fullscreen, bring up a snapshot with the arrow keys or by clicking its thumbnail, and press A. Do the same on another one and press B. Then press Tab to compare, and Tab again to come back.

Drag it anywhere along its length to slide it across the image. Press R to rotate it around its handle — vertical, horizontal, diagonal or any angle in between. Hold Shift while turning to snap to 45°, Ctrl to 15°. Left click or Enter confirms, Esc cancels and puts the bar back.

Ctrl with the middle mouse button, or Ctrl with the wheel, zooms from a tenth of the fitted size up to forty times, anchored under the cursor. The middle button alone pans, Home restores the fitted view.