Lou, thanks for clarifying re doc. I am a little surprised at the paed's comment re weaning - the 5 to seven months quoted is UK best practice so you might want to query this, or might consider asking her to clarify what her criteria for "being ready" are. You can download a copy of the BLISS guidelines from their website to show her if you feel that would help. (I would stress, I am not saying here that weaning is the answer, just that it is one of the options that you could consider).
I do think that it is so unusual for paeds to see any baby still bf at six months, let alone a 28 weeker, that their practical knowledge base is pretty limited. Telling you that three bottles a day will not affect your milk supply betrays a basic lack of knowledge about the physiology of breastfeeding, for example.
I also understand how you feel about expressing, I too had a bellyful of it. Your approach to feeding, now that the other issues re latch are resolved sounds sensible.
Don't know about the crying during a feed, however one of the things about very premature babies is that they spend more time as a newbornm just because they are here iyswim. It was a big shock to me when dd suddenly started being more alert, more demanding at around three months corrected. In your ds's case, maybe it's teething, maybe it's just he is more aware of his surroundings as he grows and develops.
Most of all, you are doing so well after all you have been through and all you have to contend with. Sending you hugs and weight gain vibes xx