Glancing through the munitions in the article, HIMARS were originally developed and deployed in the 90s, not sure if they have a backup to GPS.
Thr Excalibur system mentioned does have an inertial guidance system to fall back on but I would assume it would lose some precision that GPS would afford.
You can be marked as delinquent between 30 to 90 days (and this will negatively affect your credit) depending on the servicer. The direct loans default after 360 days which is what you are thinking of.
Nobody which is why the used market on them is crashing.