Will a new 8-slot,
You seem very wrapped up chapter and verse of complying with NEC. You're missing the point. We are not serfs to electricity. We are the masters of this technology. Electricity works for us. Bare compliance is for slumlords. Make electricity wonderful!
#1 mistake is getting the smallest panel that will serve immediate needs. This creates a nightmare later when you want to add more stuff. More spaces are a trivial amount of money, so while we certainly are interested in saving you money, chintzing out on panel spaces is not the place to do that. Go BIG or go home. Bye bye 8-space. Hello 30-space.
And if you want to save some money, avoid "QO". Eaton CH is just as good and just as compact at 2/3 the price for the breakers.
125A Square D 125A subpanel with a 125A main breaker that I want to use for a subpanel and disconnect in the shed satisfy NEC code requirements ?
(I want 2nd opinion on early advice saying I'll have to swap out 125A main for 30A main)
And if so, can you please point me to the applicable NEC code section(s) ?
Yeah, people get sucked into this mistake all the time. They get told to use a main-breaker panel as an outbuilding subpanel, but don't understand why they're doing that, and they make too much out of the main breaker, like "shouldn't that match or something?"
You do not need any main breaker at all in the subpanel. (NEC nothin'.nothin').
The only purpose of using a main-breaker panel is to provide a "main disconnect" as required in outbuildings per (NEC 225.31).
225.31 Disconnecting Means. Means shall be provided
for disconnecting all ungrounded conductors that supply or
pass through the building or structure.
However... if you plan to carry the feeder some distance within the building before it gets to the subpanel, you will need a separate disconnect near the entry point. If you do need that separate disconnect, then this satisfies 225.31 fully, and you do not need a main breaker at all at the panel.
225.32 Location. The disconnecting means shall be installed
either inside or outside of the building or structure
served or where the conductors pass through the building or
structure. The disconnecting means shall be at a readily
accessible location nearest the point of entrance of the conductors.
Needless to say, saying you need to downgrade to a 30A breaker is utterly laughable advice, the person is just "making stuff up".
Will I need two ground rods at the shed ? or just one ?
And if so can you point me to the applicable NEC section(s) ?
Two needed unless you can get free services from an electrician.
250.53 Grounding Electrode System Installation.
(2) Supplemental Electrode Required. A single rod, pipe,
or plate electrode shall be supplemented by an additional
electrode of a type specified in 250.52(A)(2) through
You can evade this requirement if your electrode tests out as having good conductivity to earth; however the cost of doing this test is a great deal more than driving a second ground rod. Other code calls out a minimum 6' distance, but more is better.
Wiring in the shed will be with (3) 12awg THHN wires (B/W/G) in 1/2" EMT. IF I run the three wires for power from a 20A breaker in the subpanel to a light switch near a cornner shed door, can I run the switched wires part way back to the light in the same EMT conduit ? And if so, the applicable code section(s) ?
You can't use green wire!
250.119 Identification of Equipment Grounding Conductors. Conductors with insulation or individual covering that is green, green with one or more yellow stripes, or otherwise identified as permitted by this section shall not be used for hot or neutral circuit conductors.
I would suggest "red". Green is reserved for safety ground, but you don't need to wire that if it's EMT pipe.
358.60 Grounding. Grounding and bonding EMT shall be installed in accordance with 358.60(A) and (B).
(A) EMT shall be permitted as an equipment grounding conductor
As far as how many wires / can you do that switch, again can't prove a negative. Certainly any number of wires or circuits are allowed in conduit, if you're willing to pay the derate in 310.15(B)(3)(a)...
However all related conductors must be together in the same conduit.
(B) Conductors of the Same Circuit. All conductors of the same circuit and, where used, the neutral and all safety grounds shall be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord...