> - No way too select grid size
The above is controlled by using the application preferences, <Ctrl> + "K" under "Units & Guide Lines" and the end of the category list.
> - Apps does not take into account Windows' default units (I use the app in French so it defaults to millimeters while my system is in inches)
The above is settable through the application preferences under "Units & Guidelines".
> - The form fields can only be boxed and not underlined
There is a property for each field, and one can set the "Appearance" of the field to include the the color, type, and size of the border. The types of "Line Style" for the field are "Solid, Dashed, Beveled, Dashed, and Underlined". The thickness can be "Thin, Medium, or Thick". You can also select the color.
Since this is User2User site we can only provide some guidance.
You might want to post using Adobe's Feature Request/Bug Report Form.
It is also possible to be invited to be an Adobe product evaluator, but rants might not help you get invited.