there's definitely signs of overactive letdown and oversupply. I've heard really good things about supplementing with Magnesium for overactive letdown, you can also try block feeding (feeding on one side for a set number of hours before switching to the other side)
I agree with Megan - hands on help would be a good idea. Oversupply can actually lead to not so great weight gain in some babies (too much lactose and too little fat, I think) but dropping supply if that is not the issue is not a good idea.
How are you breastfeeding? Do you recline back a little and have baby upright? That often helps them cope with the flow.
I had to clock feed in 10 hour blocks to get my supply under control (plus a whole bunch of other things), so if over supply is an issue there are ways to decrease it.
That sounds like heavy letdown to me, more so than reflux; usually reflux has back arching during or after nursing. If you are hearing wet burps that sound like baby spit up but nothing comes out, that is telltale reflux.
I had a pretty similar situation with my son, but it turned out to be both reflux (from a dairy sensitivity, so I went off dairy) and heavy letdown.
The heavy letdown can cause a foremilk/hindmilk imbalance (as you know) and that is often associated with green poops, and maybe the seediness, and a looser consistency. It is also consistent with fussiness during nursing and gas. I am not sure if heavy letdown alone would cause reflux; for us, that was a different issue.
I did block nursing for months, which seemed to help with heavy letdown and he gained weight great. I would just nurse on the same side until he had nursed for a total of 30 minutes on that side. It might take 2 or 3 hours for him to nurse for 30 minutes (5 min here, 10 min a while later, 5 minutes after another 40 minute spell, etc.). Then I would switch sides. Sometimes I waited longer (40 minutes on a side) if it took a while. I NEVER nursed on both sides in one sitting until ds was about 11 months old.
It sounds like your supply is fine so you could try the block nursing for a couple of weeks and see how it goes. If you think your supply starts to wane, then pumping for 10 minutes after each feeding will help to bring it back up, but that doesn't seem to be an issue for you.
Another thing is to try different ways of feeding him. For a month or so (until these other things were under control) I nearly always nursed either swinging in our sky chair (one of those hammock chairs) or else while walking around. Inconvenient, I guess, but sometimes I would stick him in a carrier to nurse and go for a walk, and we were both happy. I think that this was better for him because he was always in a more upright position, not laying horizontal across my body (as was suggested above), which helped with reflux. A rocking chair or some other motion might help.
Also, in terms of gas, there are some excercises you can do with baby if you have not already. The book Itsy Bitsy Yoga has them (a book that I love and wish I started using earlier!) as does the Infant massage book:
The one we did ALL the time was, baby laying on his back, to fold his knees up toward his chest and gently press. It was extremely helpful to ds (and eventually entertaining for us; my husband would demonstrate "farting the baby" for friends with unbelievably reliable results).
I hope that helps. It can be so stressful when nursing is hard! Hang in there and good luck!