Electronic – Controlling max voltage (power supply) when Vin varies

I agree with others that switchers are a better choice in terms of efficiency, but they can be somewhat complicated to deal with if you're inexperienced, and there can be lots of weird effects that aren't immediately obvious (precharge sinking, beat frequencies, etc.) that can make life difficult. Assuming you've figured out your power dissipation and know how much current each rail can deliver, if the linears will work for you, stick with them (at least for the first pass).

If you're trying to achieve a variable-amplitude square wave output on your adjustable rail, the chopping may introduce noise into the main 24V rail, which could show up on the other rails. You may want to have an LC filter between the main 24V rail and the regulator input to provide high-frequency isolation, and will probably need extra capacitance on the adjustable regulator output (bulk electrolytic as well as low-impedance ceramic) if you expect the square wave edges to be sharp.

1, 5) There are some dangers with your scheme.

Power dissipation in the linear regulators will be

\$(V_{out} - V_{in}) \cdot I_{out} \$

which is significant, especially for the lower output rails. 78xx-type regulators have built-in thermal protection around 125°C, and (without heatsinking) a junction-to-air thermal resistance of 65°C/W. Your thermal management will be challenging.

Another potential problem - if the series-pass element in any of your low-voltage regulators fails or gets bypassed (shorted), you'll present the full 24V input to the output. This could be catastrophic to low-voltage logic. You should protect your low-voltage rails with SCR crowbars that can sink enough current to put the DC/DC brick into current limit and collapse the 24V rail (they'll need big heatsinks too). Fuses are unlikely to be good protection since the 24V brick likely isn't stiff enough to generate the \$I^2 \cdot t\$ needed to blow a fuse.

2) Whatever floats your boat.

4) Meters aren't huge loads. Just use one of your rails.

3) Correct - all regulators have headroom requirements. If you want the maximum 24V out, you'll need a direct connection, and will have to rely on whatever intrinsic protections the brick will provide you.

A switching regulator like John suggests would work in principle, and any experienced electronic engineer would probably find it trivial, but keep in mind that that chip is effectively funneling all of that power through a 4mm square plastic package with no leads and that pretty much all the heatsinking is on the bottom (board side) of the chip. This makes the PCB design a bit tricky in that area for the uninitiated. See section 10 of its datasheet.

A less demanding solution in terms of heat removal would be to separate out some of the functions that that chip does, thus providing more surface area for the heat to dissipate, but it only shifts the PCB difficulties into avoiding parasitic interactions between parts that should not interact.

To keep it simple, I'd be tempted to go with a separate, lower voltage transformer/rectifier/filter for the smaller amps. If you happen to find a transformer that has a single primary and multiple secondaries that collectively meet your requirements, you can make it that much more compact, but those are comparatively rare.