According to my very big OED subscription, mandolin is an accepted spelling (the original British spelling, -e came from the US)
Mandoline is from the French, surely? It being a French invention. Or so I've always assumed... A quick bit of googling throws up much folklore but few hard facts, so that may be nonsense.
It probably is French, which traversed the Atlantic to the US (it's the OED, so English definitions). Actually, I couldn't remember how to spell it, so I asked the computer, and it suggested both, so I picked the one that I knew one of you would confuse. I would be suitably terrified of a woman with a silicon breast implant, of course.
Trust me on the dry salad, it's revelatory, you can't properly dress wet leaves. I eat a couple of salads a week, so it sees literally heavy rotation. It's also good for chef's bane (aka coriander, which when wet and chopped, sticks to every bloody thing, hence the name). I've never found leaving lettuce and leaves to drain to be successful. My other approach is to stick them through the cat flap and shake, but that just distributes bits of salad across the patio. A splendid Nicoise was served last night, pan-seared tuna, jersey new potatoes, steamed green beans, semi-hard boiled eggs on a bed of washed and dried and dressed spinach and some other chopped salad veg. Crisped up some capers in the pan, added tarragon, a little dijon mustard, and oil. Let that cool, add creme fraiche, and then dress the salad with it. The only thing that would have improved it was eating it outside a cafe in the south of France rather than our living room.
The all-round chef's knife I use is from Sheffield too, it's been around for a while as I bought it when I lived in Sheffield.