The circuit is fun to follow but very very very unconventional.
If you submit circuits like that Olin will have a triple fit and explode on the spot :-). The zig zagging layout style and having current flow and control "flow" in both directions makes the diagram very hard to follow, and the bits that you have left out also do not help.
BUT
Most of it seems possibly OK.
The Pololu buck converter information here accepts a wide range of inputs voltages and has an adjustable output voltage of 4 to 25V at 300 mA max.
The drive to M1 is ingenious but not certain to give good results. M1 should have a gate signal of no greater than 12V when it is turned off and a gate signal of between about ground and 6V when turned on. Your arrangement achieves something like this but relies on the supply rising to 37V when the LEDs are off (so 1/3 of 37 ~= 12V. and when LEDs are on and Vload supply ~~~= 24 V Vgate = 24/3 = 8V. This gives 4 Volts of gate drive which will work for some FETs BUT it is based on an ill defined loading of the power supply.
The FET that you have chosen datasheet here is a good one and the gate threshold voltage about matches your control method. It will probably but not certainly work in practice.
A more reliable method would be to use the PIC pin 7 to drive a small "jellybean" npn transistor - say 10K from PIC pin 7to NPN base. NPN emitter to ground. Then connect NPN collector via a say 1K to P Channel FET gate and connect a say 10k to 100k resistor between P Channel FET gate and source.
Supply FET source from 12V from buck converter as already planned.
The FET is now driven by the PWM signal and you will also get dimming of your 1 Watt 12V LD strip if PWM is applied to the original LEDs.
If you do not want dimming you can add a say 1 uF or so to the NPN base and a diode across the base drive resistor. The capacitor will be charged via the diode and discharged by the resistor when the PWM stops. The capacitor on the base will smooth the diode provided PWM so that you FET stays on during PWM. Fine tuning of cct required. Out of my head - probably OK but values may change.
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.
Best Answer
IMO it would make much more sense to use an SE-600-24 and use buck convertors to create a local +5V supply for your LEDS.
You have the potential to need conductors that will support many 10's of amps (if you have 4-5 major wiring runs) if you use a 5V supply and it is best to get this done at the highest voltage and lowest current possible. Running 5V any distance you will end up with significant voltage lose, and may end up with signal ground problems for your LED data.
There are plenty of 2-5A Buck convertors like this, this (I've used a bunch of these and I like the input capacitors) or this available at low cost that would support groupings of up to 50+ of your LEDs allowing much smaller wires to be used in your installation.