Can last years used multi purpose compost be added to kitchen and garden waste in compost bins to reenvigorate it, or would it be better to dig it into my raised beds?
Perfect gardening practice involves using fresh compost for potting and seed sowing every year. Older compost will have lost some of its nutrients. If the bag has been open, it will have gained extra micro-organisms - that's why you often see white fungus in the bags. However it still has many excellent uses and you have named two. Personally I'd probably put it in the raised beds, especially if you have soil that lacks organic matter (i.e. clay or sandy soil). Dig it well in. It would do fine in your compost bins, but that would be a bit of a waste.