[snippage]
I am now pondering my silly collection of MOBO mounting screws.

https://www.aliexpress.com/item/32957035974.html

Shows nylon screws that can be used for mounting or standoffs. Standoffs
don't work for me because I need PCI VGA card.

Size:
M2 (Thread 6mm)
M2.5 (Thread 6mm)
M3 (Thread 6mm)
M4 (Thread 6mm)

I guess 'size' is pitch, because all are 6mm thread.

What size would work?

Length:
5mm (50pcs)
6mm (50pcs)
7mm (50pcs)
8mm (50pcs)
9mm (50pcs)
10mm (50pcs)
11mm (50pcs)
12mm (50pcs)
13mm (50pcs)
14mm (50pcs)
15mm (50pcs)
16mm (50pcs)
17mm (50pcs)
18mm (50pcs)
19mm (50pcs)
20mm (50pcs)
21mm (50pcs)
22mm (50pcs)
23mm (50pcs)
24mm (50pcs)
25mm (50pcs)
26mm (50pcs)
28mm (50pcs)
30mm (50pcs)
32mm (50pcs)
35mm (50pcs)
40mm (50pcs)
45mm (50pcs)
50mm (50pcs)

What Length would work?

The trick is not to lose the hardware that came with the
computer case.

The standoffs are custom length, intended to elevate the
PCB to be flush with the slot holes for the I/O cards, in
the back. There's no way for me to guess any such measurement
from here. There are as many styles of solutions, as there
are days of the week.

If you artificially elevate the motherboard, with too-tall
standoffs, then your video card won't fit.

It's one of the items on the buyers list, to check the
fit and dress of standoffs before buying a computer case.
And that's pretty hard to do. You can't always inspect
this issue in advance.

I used to look at computer cases at a mom&pop (single store),
while they were in business. Basically, any store in town that
put computer cases on display in a useful way, went out of
business. And I did buy cases from them, so it's not like
the effort on their part wasn't rewarded.

For example, I don't buy "bump" style trays. That's where
the metal of the tray is formed into a bump of the appropriate
height, then a hole drilled, then the hole is threaded. While
it saves the manufacturer $0.0001 per hole, it doesn't do me
any good. Instead, I like the brass standoffs that have a
male thread on one end and a female on the other, then the
motherboard screw fits to fasten the board down. These have
a better controlled contact area where it touches the
ground ring on the motherboard.

In terms of "places that short", there was one Asus motherboard
where the plastic socket frame around the CPU socket, was
fitted *before* the soldering step. The plastic went through
a wave solder perhaps, and it swept up some solder
underneath the plastic (hard to inspect) and it
shorted out VCore. But that would only cause VCore
with a current limiter to trip off. And a little repair
work, you could find and remove the short.

But for direct rail shorts, sometimes there can
be a short inside the motherboard, but it's less likely
there is a short, say, right at the 24 pin connector.

Some Biostar motherboards, used to join the yellow 12V wires
on the 2x2 ATX12V, to the 12V wires on the main 24 pin connector.
This is not a good idea, and I have no idea why Biostar was
doing that. The 12VA and 12VB should be separate from one
another, in case some old PSU with actual separate transformers
for those two 12V was in usage. The 12V on the ATX12V 2x2, could
be a different 12V than the 12V on the main connector (fraction
of a volt difference).

Since you say you are observing a short with the ATX12V removed,
then this suggests something on the rails on the main 24 pin
is shorted. But I don't see a cosmetic root cause, because the
transfer of some of those rails around the motherboard, is on
a buried layer. The motherboards have 4,6, or 8 layers, with
6 layers used for RAMBUS boards, 4 layers for a lot of regular
DDR motherboards, and 8 layers reserved for sockets with
huge pincounts, where it's just too hard to route all the
signals using a limited number of layers. Somewhere in
that stack, can be some 2oz layers for routing power.

At work, we would use a bed-of-nails tester for evaluating
"ohms". After a board came off the line, sometimes tests
were done that way too (JTAG), which isn't all that common
in the computer industry. Those are known as structural tests.
A company like Asus, would use a camera and automation to check
for visible flaws (stuff in places it doesn't belong), can
use an Xray machine for BGA verification, but for electrical
test, there is only a 2 minute hand functional test
(lady plugs boards into mobo and runs some test code for
two minutes total time, then product goes in box). They
rely on statistical sampling for quality, rather than
every board being inspected with a magnifying glass. Every
board receives the 2 minute functional test, but other
steps are likely automated.

To visually inspect a square foot of PCB, takes about 2 hours
with a 10x magnifier, because I've done a few boards visually,
and you do find stuff. (That's where I found a certain brand
of resistors, with all the bodies cracked in half.) At one
time, Asus made 5 million motherboards per month, and there's
no time for that sort of inspection, and they can only
sample motherboards and inspect them for manufacturing
defects, then go back to the line and correct them. Not every
motherboard can be treated like a Rolls Royce.

Thanks Paul

From Google:

Thread: What size/kind of screws do I use for mounting motherboard?

Screw size depends on the size of the threads in the mobo standoffs, usually
M3x0.5 (metric) or #6-32 (US standard). I personally would not go over 6mm
or .25in in length. Head configuration is entirely up to you. Most
well-stocked hardware stores should have just about any size and type you
want.