if I compost some of my seeds, and leave them in my inventory, will I still be able to use them for next year's spring festival?

most likely they'll just turn into tarnished eggs (expired springfest currency)

Generally no.

Items from one festival are generally not useable in following festivals, whether or not the feature returns. This is to prevent hoarding and sudden influx of items. So you'll always need to work for each festival's items, and then use them before the end of the festival.

Whatever is not used is usually converted to some form of expired currency the following year, that has no purpose other than being able to be sold for a few gold.

Ok thanks!

Thanks!