Yes, absolutely replace it. When you tighten the plug, this alu washer gets crushed. You will feel when it's slightly harder to turn the ratchet - that's when the washer STARTS getting crushed. The washer accepts a certain amount of crushing then it becomes 'stiff' - it no longer crushes. If you try to reinstall it and it's stiff, it won't seal properly and the oil plug may also not stay tightened from engine vibrations. MY SUGGESTION - when you feel the new washer starts getting crushed, rotate the ratchet less than 1/4 turn then stop even if it doesn't feel fully 'tight' on the handle. Also careful as latest cars are now using aluminum oil pans, you don't want to strip the pan threads by forcing on a stiff washer
Its a good practice to change your crush washer every oil change. However, its not that critical. If you ran out of replacement washer, you can reuse the old one and just change it on your next oil change. Just check the drain bolt after a few hundred miles to make sure its not leaking. If it is, tighten it a little more.
Not here. My guess is it's to stop the plug damaging the surface of the sump when tightening. Their definately steel and come in the packet with the new filter OE and aftermarket. They may use alloy ones in some other models but I haven't seen them. I don't use them. I use alloy ones which I suppose is a personal choice. Old school logic.