if the wool is over-dyed felted wool- you do not need to re-wash it.
i have never had any luck with fusables --- so i don't waste them on my wools.
i just pin-or hold in place to stitch- but i've had lots of practice- a glue stick may work for you-i've just never managed to make them work for me.
i generally use a chalk marker of somne sort- since it's easy to brush it away.
i do wash my finished projects- because i make things that get used-and laundered when needed- if you make wall-hangings- items that dont get laundered- then i guess you wouldn't need to wash your project when it's done. i have never had a problem washing/drying a completed wool project-and i've made wool quilts, table runners, candle mats, penny rugs, floor rugs...i work with wool alot...my wool appliqued quilt has been washed a number of time (and dried in the dryer)