My first response would be to question whether the players know every single thing about their characters, even after the number of sessions that you have played with them. I find it very difficult to believe that there are not gaps in their backgrounds that you can use this as an opportunity to flesh out.

Looking at the list you've provided, it seems clear that these are written to be used by new characters, so my approach would be to adapt/change/use these as inspiration for bonds that **do** make sense for the characters you have, so that there is something to tie them into the adventure. 

Take 4. as an example - *surely* there is one of your PCs who could have a link to a similar NPC. Similarly for 1 and 5.

I would try to view these bonds not as a limiting straight jacket, but as a tool for increasing the richness of the character backgrounds. Also, if possible then get the players involved in coming up with these bonds. There is a similar mechanic in Dungeon World and Apocalypse World where the GM is specifically encouraged to ask questions to the players to establish these types of bonds.


As for why these things haven't come up so far in the PCs' adventures, there could be any number of reasons , which is exactly why I suggest discussing and coming up with something plausible with the player in question. Maybe the reason a mentor hasn't been in touch is because they've been ill. Maybe he's been resisting contacting the PC for some reason, maybe he doesn't trust the PC for some reason but his hand has been forced etc. 

So ask, 'which of you has a link to Greenest? Why?' etc. You will be surprised how creative your players can be, and getting them involved will give them a real buy-in to the story. If you do right, you're also likely to end up with a bunch of character specific plot hooks for future adventures that you can use to make the campaign much more personal to the party.