Hi, Alain! :wave
I'm not familiar with the cushions Trillium
uses, but I assume you think you can get them into your washing machine. If so, I think you're on the right track. The spin cycle of the washer would be good for getting the water out (the biggest problem with trying to launder by ''hand'').
I don't think shrinkage will be a problem if you use cold water... although I wonder if the cushions will retain their shape very well. (Probably, but no guarantees.) Can you not remove the covers and just wash them (on gentle cycle)? I really think with cold water you will be okay; probably would also be okay to throw the covers in the dryer on low heat... but line or flat drying is surely safer.
Do test the fabric before washing, to be sure its color won't bleed. (If it's a synthetic, which is typically used, no problem. If it's wool or some other natural fiber, though, you better check it.)
I tell ya, washing the cushions in my Casita
would be a big
chore... which is why the others are suggesting the spot treatments. But, if you need to wash the whole, entire cusion(s), I don't see why your ideas wouldn't work.
Unless it takes forever to dry?