Electronic – Ensuring common ground in a circuit with several voltage requirements

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.

I have similar project.

I did it with two voltages. First I put the switching regulator to drop to 3.9V for GSM. Then LDO to drop from 3.9V to 3.3V for MCU, GPS, CAN.

I see the GPS module you pointed out possibly has it's own LDO on board and that's why its voltage ranges from 3.3V. Take a look on those modules alone http://www.gtop-tech.com/en/category/GPS-Antenna-Module/A01_MT3339.html they all go from 3.0V so both voltages could be used (3.9V or 3.3V).

For CAN tranceiver I used SN65HVD230D from Texas Instruments, which is a little more expensive but works with 3.3V (that eliminated necessity of third 5V supply).

Don't use voltage dividers for supplying power. The load connected to this divider acts like another resistor in parallel with one from divider. If the load you connect is mcu or some module, the impedance is unknown and variable in time - you wont be able to achieve desired voltage.

Voltage dividers are good if you connect some high impedance line to it. Then almost no current flow through that line and the voltage at the divider stays at the level it was set. This should be fine in case you need to lower some logic levels (you mentioned 4050 which is a good solution).

You might also need some protection circuit at the power input, before the voltage regulator. Automotive power can have load dumps (100V on power line for about 400ms), positive and negative voltage transients. Cold crank condition can lower the voltage to 3V for 15ms. Jump start can raise it to 24V.