For question 2, the Output section is the same as in Max's native Bitmap, and information can be found here:
https://knowledge.autodesk.com/support/3ds-max/learn-explore/caas/CloudHelp/cloudhelp/2016/ENU/3DSMax/files/GUID-9E9C1FBF-6C6F-4418-AE0F-92EB0D5C8139-htm.htmlI never head in there except when I am using a bitmap purely as Self Illumination or an HDRI for the environment, where I use the Output value to control the brightness. For a regular material with a bitmap in the Diffuse I never normally need to use those parameters if using a regular PNG, JPG etc.
For question 3, the tooltip pop up describes it well - the value shown in your image 1) is how much the surface reacts to light, while the value shown in 3) is how much the colour from the bitmap is blended with the base color for the diffuse (prior to then being affected by the Level value saying how that final colour will respond to light). So if you have a bitmap, a Diffuse Color of red, a Diffuse value in the Maps rollout of 0.5 then your colour will be a 50% mix between red and the colour from the bitmap image - this resulting colour will then be affected by light in the scene as controlled by the amount set in the Diffuse Level.
Hope this helps!