Ah, the UK! Now the vaginal breech birth makes more sense. The US is all about the obstetricians, and midwives are considered a fringe choice.
I'm sorry you can't get help around the house. It is so hard to balance pumping and baby care. One thing to remember- this is a short period in your baby's life. If pumping means that sometimes he fusses because you cannot immediately run to him and pick him up, that's okay. He's not going to remember.
Having too much milk makes the challenge of mastitis much more difficult to deal with. You are correct that emptying the breast is likely to perpetuate or even exacerbate oversupply, and that can in turn make mastitis more likely to occur. But when you're dealing with oversupply and mastitis together, the rule is to treat the mastitis first, beat it, and then work on reducing supply once the infection is gone. Therefore, I highly recommend pumping the affected breast frequently, emptying it as completely as you can (massage will help) until the mastitis is 100% gone. Once it is, start allowing more milk to sit in the breast in between feeding or pumping sessions. Don't let yourself get engorged- if you have a tendency towards mastitis you want to go slow, and not let too much milk build up in the breast.
Now, as far as the non-mastitis breast goes, try to empty it a little less aggressively than you are doing right now. 4 oz at a time is a lot to take out at once. I think you might do well to try to stop at 2-3, particularly because it sounds like you have absolutely no shortage of milk right now, and no need to stockpile vast amounts.
Because milk is continuously being made, the breast will never really empty out, but if you can pump for a couple minutes past the last drops you have probably done quite a good job of getting it emptied.
I also strongly suggest that you be more rigorous about maintaining some sort of pumping schedule, particularly for the infected breast. When a mom with oversupply is able to nurse, she is generally advised to nurse frequently. Allowing the baby to take smaller amounts of milk more frequently keeps the breast from getting too full and uncomfortable, and just moving the milk through, albeit slowly, will likely remove the risk of plugged ducts/mastitis taking hold. I'd love to see you pump every 2-3 hours throughout the day. Every 4 at night is a little longer than I'd want to do- but you need your rest and should feel free to take those 4 hour stretches if necessary.
Fixing a difficult latch is sometimes easy, and sometimes it's just about impossible. You just have to wait for that tiny mouth to grow. Did anyone show you the "nipple sandwich" technique? That can sometimes help you cram the maximum amount of breast into a tiny mouth.
ETA: 3 possibilities forlong feeds- either he's a big-time comfort nurser, or he's just a slow (maybe sleepy?) feeder, or his latch is so poor that it takes him a long time to transfer adequate milk.