If it were me I would use your flag - for the simple reason that it is your flag. It is imbued with your experiences and should, quite rightly, follow your career.

Your mother has put you, unfairly, in an impossible position and given you a guilt trip. Not wishing to be morbid, but if you feel strongly enough, you could always stipulate the use of your grandfather's flag for your own funeral. It would then create a family history and heirloom. Rather like some families have a christening robe, yours could have a funeral flag.

Civilians can also be patriots. There is the famous quote, "They also serve, who only stand and wait." In the UK, many non-military people are buried under a Union Flag. Is that not possible in the US?

Regards

Chas