I understand that this subject has been analyzed and debated to death here over the years. I wasn't around during that time (as I'm one of the people whose interest renewed with the reunion), but I've read most of the GnR related stuff that's available out there recently, so it's fresh in my memory.
I haven't seen any other quote by Axl besides the "no lyrics, no melodies, no changes, sing what I was told or fuck off" one from the 2008 chats, when he was referring to the 1995-96 period and the Snakepit songs.
As for Duff, the only Axl quote I know of referencing him in a "been put down/insulted by" context is this one from 2013:
Supermodel girlfriend Stephanie Seymour and original Guns guitarist Slash and bassist Duff McKagan "did more damage to my ability as a writer. To those three, it was all crap. It beat me down so much. At the time of the (Use Your Illusion) tours, Slash and Duff said, 'You're an idiot, you're a loser.' I didn't write for years.
He was referring to a different situation and time frame though, and I don't see how it could be associated to "Sorry". Moreover, Duff couldn't have said something along the lines of "shut up and sing" during the 1995-96 period, as he had teamed with Axl about the Snakepit songs.
I think that the Sorry lyrics are situations specific, but not just one person specific; some parts could be addressed to one person, some others to a group or groups of people; or "you" may be one character that combines elements of different people. The biggest part of it doesn't make sense to be about Slash at all in my opinion. I don't have time to write an in depth analysis to support my interpretation (it would be O/T anyway -it already is- and probably pointless and boring to many people here who have read about it and discussed it multiple times), but for example:
You talk too much
You say I do
Difference is nobody cares about you
You've got all the answers
You know everything
Why nobody asked you
'S a mystery to me
The first line could be about Slash, as he was talking to the media a lot, but it continues with "you say I do"; I don't think Slash ever accused Axl of talking too much and Axl has never alluded to something like that. Then "difference is nobody cares about you", which could read as "the difference is that you/they care about what I have to say, but I/nobody cares about you (because you're a nobody/not important). "You've got all the answers/you know everything" --> irony: you talk like you knew everything, even though you're clueless (I don't think he could say that about Slash). "Why nobody asked you..." --> same as above: nobody asked you because you're a nobody.
I don't see Slash anywhere in this part. It mostly recalls what Axl was always saying about the media people (see also "I'll kick your ass like I said that I would" --> Get In The Ring).
I believe (based on what has been posted here and elsewhere) that Sorry was one of the last CD songs chronologically, probably written during a period (early to mid 00s - the lyrics maybe a bit later) when Axl spent a lot of time on the internet (which he had called a "garbage can" or something) reading what fans and random people (="nobodies") were saying and speculating about him. Also at the same period there were articles based on "inside information" about the then present but also digging up his past and personal stuff he'd never want to become public for obvious reasons, with some of the anonymous "insiders" being referred to as "friends".
The lines "You know where to put your/just shut up and sing" could very well be a sum-up for everything that was said about his rants ("don't talk, just sing") by the media and a portion of fans, as well as for the pressure by the label and fans for not finishing the album (there is a post in this forum from 2006 or so by a fan who had met Axl back then, and what Axl said there relates a lot to this sentiment). And what precedes them ("Nobody owes you a goddamn thing") reminds of Axl's "I don't owe you/I don't work for you" online rants addressed to fans.
Of course there are lines reflecting a more personal relationship (e.g. "You choose to hurt those that love you", "You've got not heart/you can't see/what you've done for me"), but they don't have to be about Slash; they could as well refer to people Axl thought as his friends who "betrayed" him by talking to the media or in other ways (e.g. by not supporting his new band - he has talked about that too).