Well, let my try to answer all that. Since I don't buy a lot of buddy (companion) passes, anyone please correct anything I get wrong....
1 - Yes, you can cancel one listing and make another online.
2 - can't do buddy passes online, only those on employee's travel card. Buddy passes must be purchased at the ATO (or CTO if any still exist).
3 - The buddy passes I've gotten have been for non-eligible family members, so I've paid for them. Don't know if the employee has to purchase or if the user can, but presume that the employee must.
4 - Other than when international travel is designated as such does the employee have to travel with the buddy pass user - generally "in season", but it's announced. Except for that, the buddy pass holder can be on their own while traveling (though as I said, I think the employee has to actually buy the ticket).
Actually, the buddy passes haven't been that great a deal if you can plan ahead to take advantage of lower fares. The price has been 10% of full coach fare plus all taxes/fees. As I said somewhere above, in Jan it was $750 for two GSO-MAN R/T. We didn't decide to go till about 3 weeks prior, but even then we could have gotten regular tickets for about $850 on DL or UA. But with my seniority, our seniority based boarding priority, and a desire to travel together (we were renting a car and driving about 2 hours to our ultimate destination) we went the buddy pass route. If it had been just the youngsters traveling, I'd have bought a regular ticket. That's where the new "guest passes" are a big improvement - price.
Jim