There’s no reason that guilt would be absent from helpinghelp a specific person in need (like your struggling mother example). Plenty of people feel guilty taking handouts and will outright refuse help when they might need it.
As for the drive thru thing, I think you might be talking about something different than what I’ve seen/done, which is just paying for your own meal and the people behind you. There isn’t any expectation for them to continue some chain, and in many ways it’s a bit of an empty gesture (they are just taking that first person’s goodwill and passing it to the next in line).
My interpretation of paying it forward is the premise of receiving something when you’re in need, then, when you’re able, to give something back. Not to the one who helped you, as that would be repaying a debt.
While in principle, I don’t disagree. If you’re impaired, you shouldn’t drive. I lost a parent after they were hit by a drunk driver.
However, there are monstrously different amounts of impairment. You have reaction times and motor skills, decision making and judgement, awareness and attention.
Implying any type of impairment to be an unequivocal “no” to driving is short sighted, in my opinion. It’s the easy argument to point at any mind-altering substance: caffeine, tobacco, or antidepressants could be classified an impaired driver.
It’s also worth pointing out that even different emotions could dramatically alter driving performance. Not that we would ever think about restrictions on crying while driving.