A pay-it-forward scheme works like this. Person A does a good deed for Person B and just asks that, in return, Person B does a good deed for someone. Person B does a good deed for Person C and just asks that, in return, Person C does a good deed for someone. The cycle continues like this, with the goal being to create a flowing river of good deeds.
Pay-it-forward schemes have been a trope for a very, very long time, there was even a bad movie made about it (oh Hollywood, what would we do without you). However, as even the movie acknowledges, they are notorious for eventually fizzling out. If you had the authority or whatever that would allow you to and were to ignite a chain for as long as possible, how would you do it?
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.