If potential is “to be able to meet”, then I’d say past potential would be “to have been able to meet”.
This is conveyed as a concept of “to have been able to meet” by having it modify the noun こと (which is used for a “conceptual thing”). This then allows the concept to be used as the が-marked subject (since only a noun can be the subject).
According to this book https://en.wikisource.org/wiki/Things_Japanese “The probability is that two distinct words— maru and maro —have flowed into one, and so got confused. […] The word maro, on the other hand, is an archaic term of endearment.”
So it could also possibly be 幸福 combined with a suffix of endearement.
Any ideas about the first names old man is mentioning?
ブチタ - I feel like I’ve heard this before but I don’t remember / don’t know what it could mean
マメ助 - I found that まめ can be used as a prefix for “miniature; tiny”, so maybe the meaning of this one is something like “tiny helper” or “tiny rescuer” (like Fukumaru being someone who saves old man from being lonely?)
おむすび - pretty sure this means rice ball (similar to onigiri), with that black spot on his head, Fukumaru’s head does look like a riceball!