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I can't view the images.
I would go scorched earth on the cupboard. Bicarb fucking everywhere, air purifier, vinegar spray, sugar soap, dehumidifier, fans, open windows, fabreeze, bleach. Basically soak it in whatever level of chemical cleaner you are happy with then suck all the moisture back out.
We have had an air purifier and dehumidifier running since day 1 here. I think they have helped with the smells and with our heating bills.
Yup, all soft stuff removed, all ceilings cleaned, and then plastered, all wallpaper and lining paper removed, all wooden floors sanded and sealed, all skirting boards sanded back and painted or replaced, all doors sanded and repainted. As I say it is just this one small cupboard that has retained the smell, the rest of the house is fine, I'm not sure what we did differently but here we are.