Not knowing the condition of the paint, it is hard to make solid recommendations. The one thing I might try based on what info you gave is to see if Power Finish on an orange pad gives enough cut before hitting it with the D300 and microfiber.
The general rule of thumb is to use higher speed and more pressure for more aggressive correction. I use 6 on my PC and work my way down, usually finishing between 4 and 5 on a three stage correction.
As for your final step, my only suggestion is to try it out. You always want to use the least aggressive correction you can to acheive the results you want. Try the grey pad and if it is removing everything from your previous step then that is the combo you should use. If you still notice very light scratching or hazing, step up to a more aggressive pad.
For glass, I use a special glass kit. It comes with different polishes and special disks. I've never tried glass correction using the products I use on the paint, except for Klasse AIO to clean and seal. Perhaps someone else has experience with these products on glass.