Becky, two separate answers here, and the second one matters more than the first.
On how long: the "few days to a few weeks" line is about the feeling of fullness settling, not about milk disappearing. After a gradual wean it is normal to leak a little for several weeks, to be able to express milk for a few months, and to get the odd drop for many months after that, sometimes a year or more. Three weeks in, still leaking at night, is exactly what most of us had. Production winds down on the same supply and demand principle that built it: the less milk is removed, the less is made. It just does not switch off like a tap, and there is no medicine or trick that speeds it up that I would put in writing here.
On the walnut: that is the thing to take seriously. A hard, tender, warm patch during weaning is the classic blocked duct, and yes, you absolutely can get mastitis when you are no longer feeding, because the risk was never about the baby, it was about milk sitting in a bit of breast that is not draining. Weaning is one of the most common times for it, especially when the last feed ends abruptly, as yours did. Gemma's routine is the right one: warmth, gentle stroking towards the nipple, hand express or pump just enough that the patch softens, then cold. The trick is the word "enough". Removing a little relieves the duct without telling your body to keep production up; emptying the whole breast does the opposite. The site's guide to blocked milk ducts walks through it step by step, and the stopping breastfeeding piece has the section on keeping comfortable while your body catches up.
When I weaned my second, I dropped the last two feeds in the same week because life got in the way, and got a tender patch exactly like yours. Because of my own mastitis history I was watching it like a hawk, and it went in two days with the warm-then-cold routine. The line I held, and would hold for you: if you get a fever, flu-like aching, a spreading red area, or you just feel unwell, that is a same-day call to your doctor rather than another night of pressing it. Mastitis needs looking at if it has not settled within 24 hours, and you already know what that fever feels like.
Cabbage leaves: cold, and they fit in a bra, so they do a job as a cold compress. There is no strong evidence the cabbage itself is doing anything a cold flannel would not. Use whichever you have. No binding, no scarves, no tight sports bra either.