I would think you could sterilise the feathers in steam although I haven't done it.
Fleece is unlikely to survive being washed in a washing machine, even inside a pillowcase, without felting into a big ball. You might get away with some breeds but most not. You won't be using much fleece in art projects so washing by hand is quite quick. Just take a small amount and sink it into a bucket of hot water with a dash of Ecover washing up liquid, Lakeland 'woolwash' or similar. Leave it to soak for 15 mins, then scoop it out carefully, without squeezing or swishing too much, and sink it into another bucket of clean warm water to rinse. Finally lift out carefully, squeeze without twisting, wrap in a towel to dry, or you can spindry it at this stage. Finally lay out to dry flat. If your art project involves keeping the staples intact, then hold a few in your hands, don't let go of either end and just swish them gently in the hot soapy water then in the rinsing water, then lay them out to dry, all without disturbing the organisation of the fibres. For felting, you will be carding anyway so jumbling it all up a bit doesn't matter. Your main concern is not to let it felt before you have started work on it - with felting you use a lot of soap, high temp and shocking of the fibres with heat changes, rubbing and impact, so when washing, try to avoid all those things.
HTH