It depends upon the individual school how difficult the waiver would be. Some colleges are less strict than others. You can always try.
Grandmother’s finances would not come into the picture. Just the father’s. Yes, it is possible that you would need proof of non filing of 1040 on part of father. The IRS does that upon request. The schools can ask whatever they want and some more probing than others.
Please do not let these posts that have road blocks stop you from attempting to do things like trying for a NCP waiver. These are things that may come up. You may get a waiver from some schools, not others, and some schools may ask for more things in order to get the waiver.