Before I start buying bits and pieces, is all the Shimano 9 speed drivechain stuff compatible with each other? Specifically a road mech with a MTB shifter.

Yes
(well at the rear anyway, front can be hit & miss)

I run it the other way around on my winter road bike.

Mtb front and rear mechs with road shifters.

So it should work.

Great thanks everyone.

Follow-up question:

7 speed shifter, 7 speed cassette, 9 speed rear mech. Assuming I fiddle the limit screws so the mech won’t swing past the end of the shorter mech, that should work right?

Should be fine. It’s only really when you go beyond 9sp or earlier than 7sp it gets really messy for Shimano cross compatibility I think.

Yes, 7/8/9 speed Shimano rear mechs are all interchangeable as long as you have the correct shifter for the number of gears on the cassette.
the actual difference in the width of cassettes is negligeable between 8 and 9 speed and only a small difference to 7 speed so nothing to worry about there.

7 is quite a bit narrower. 1 eighth in fact, about 3.5mm. But yes, all the rest of that’s true

Yes, I’ve discovered that 7 speed cassettes are narrower than everyone else, which is a pain as this wheel has a 7 speed freewheel on.

Glorious plan involves a 9 speed mech for some level of future proofing, and 7 speed shifter. When I get the wheel replaced we can also swap the cassette and shifter.

Thanks!