While installing a Tange Levin threaded headset, I discovered I either need to cut the fork's steerer a few millimeters, or add some spacers. I only have the one small slotted spacer that came with the headset, but I do have some unslotted 1" spacers. Can I still use these? Would it be better to position these above or below the single slotted spacer?

Slotted spacers should be tossed and replaced with non slotted ones. Your LBS should carry many 1" threadless spacers that will work.

I just wanted to be sure I wasn't missing some critical reason for the key.
Oh I've got them. None in silver, but it's not a bad look.
Those, along with axle spacers, water bottle bolts, spare seat binder bolts, and bags of balls are among the things I've learned to just "have", whether I'm in immediate need of them or not.

Don't toss the no-turn washer. That's bad advice from someone that doesn't understand its function. I'd put the spacer above it.

I kept it on because it doesn't simply fit on top of the adjusting cup, but fits into a shallow recess on the top of the adjusting cup. I checked Sheldon's site, but found no real explanation for the key and slot.

After a few years, my Tange Levin decided to shrink. That's the only way to describe how it happened - one day working fine. The next it was to short to ride the Roller-Coaster. So I found an old Campy spacer. And it's been fine ever since. All 26 years. Them Levin's ain't no lemons.

Can a headset develop osteoporosis?

Maybe your fork grew. They go through spurts. I pack mine with Marlboros. Smoking stunt the growth.

You have no idea what you're talking about.

The only thing that keyed washer does is end up stripping because it encourages people not to use the proper headset wrench and proper top nut wrench to tighten their headset. A keyed washer serves NO good purpose WHATSOEVER on a headset that has wrench flats on both the cup and the top nut. Its original purpose is to allow tightening of the topnut without using a headset wrench below it. Which it fails, and fails very good at doing.

I understand exactly what the purpose of a keyed washer on a threaded headset is and why it shouldn't be used.

*You* obviously understand the first part, but not the second. Educate yourself.

That is the Tange headsets greatest advantage - the low stack height. Most of the new shimano 1" cartridge threaded headset have much higher stack heights than headsets that originally came with road bikes of yesteryear. And the fork was cut to that stack height with minimum amount of spacers.

When I was building up another bike with very little steerer available, I asked around for suggestions on low stack height headsets. Tange and King were the most popular suggestions. I got nothing but mixed messages on Shimano units. There seemed to be a lot of confusion between models and years.
The Tange wins out over the King on price four times over.

The *old* shimano threaded units had a low stack height. All the modern cart ones are higher.

I never had one "fail very good at doing" in 40 years of wrenching. I think you need to go back to school, kid.

To answer the question, you can use a regular spacer/washer with the keyed spacer. Just use two wrenches to properly adjust the headset.

Well, unfortunately in your 40 years, you've neglected to acquire any sort of apparent skill. Would you like to remove the foot? Or shall I let you continue? Next time you want to insult me you better make sure you are 100% correct before posting garbage.