Coasting down hill in neutral

As long as the oil pump is performing to spec, you don't have anything gumming up the oil passages, and your oil, engine and cooling system are in good shape, then idle speed should be just fine. Lots of people make boneheaded mistakes like leaving the keys locked in the car, in the ignition with the engine running, and idle speed has not harmed the engine long-term.

Even so, I still wouldn't recommend coasting in idle though. Engine braking is supposed to be something that you use to your benefit in downhill grades, and the ability to quickly brake and then quickly accelerate again (as such situations may require) would be hindered if you're just coasting in neutral.

Probably, especially at higher speeds. At 70, you're asking the engine to instantly rev from idle to appropriate revs for 70MPH. Even on overdrive, that engine speed will be significantly higher than idle. Meanwhile, you're forcing the transmission to suddenly drop into gear and deal with the difference in velocity vs. difference in engine speed.

What I meant to ask is does the idle speed provide enough lubrication for the *transmission*. Since parts of the tranny are still rotating the same speed as if I was in drive and going 70mph.

Like I said, when going fast, I give it a little throttle before putting it back into gear. (similar to shifting a stick shift without using the clutch) So although there is an almost instant change of rpm, it's not at all like going from idle to 70 mph revs.