I have an 2000 Schwinn Homegrown that hasn't been ridden or maintained in a number of years. The headset (their integrated 'ICBM') is tight not moving very freely. Hopefully, it just needs cleaned, regreased and adjusted, rather than replaced.

Last night I tried removing it, but didn't get very far. I removed the cap bolt and cap, the stem, and a couple of spacers. Beyond that, everything seemed welded.

Should the top seal of the headset just lift off? Because it doesn't want to. I tried giving the fork steerer a couple of knocks with a mallet, but it didn't budge. If it's all bound, should I just hose it down with WD-40 to try breaking it apart, or is there a better way?

i think you have the right idea about how the headset should be removed. yes soaking it in some type of penetrating oil might help. at least that's what i would try.

Normally, would the top seal remain stationary, or does it turn with the fork? Right now it's turning and seems bound to the steerer tube, but maybe that's just because the stem and cap bolt have been removed.

One thing that was a little worrying was that the spacers (3 x 3mm) were difficult to slide off. I hope that's not an indication that the steerer tube is out of round.

In a threadless headset the bearing is kept in place by a split tapered cone. On some designs the top cap sits above this and is the driver with the cone fitting a bearing race below. On others the cone holds the top cap, which has the bearing race integrated into it. Sounds like yours may be the latter. You can confirm by looking at the center of the cap, and seeing if you can spot the top of the centering cone sticking out slightly.

Now, these centering cones get driven in fairly tightly, and with a bit of corrosion, or just plain friction can stay put even when the pressure is off. Try giving the fork a firm shot downward from the top to drive it down a hair and create a gap the cone can pop up into. (don't get carried away, use a block of wood rather than a hammer so you don't damage the top of the fork). Once there's some paly in the headset, a gentle tap on the top bearing usually separates it from the cone.

Also, try some penetrating oil (make your own with a few drops of (any) oil of some kind mixed into acetone or nail polish remover, and apply at the center of the top cap, on both sides of the cone. Give it a few hours to soak in then tap the top of the fork to pop the cone loose.