Starting off my first Solar setup, got a few questions.

[PVorBust]

"Every Watt they can get" has trade-offs, and those need to be considered.

Mixing a random assortment of panels has it's trade offs too.

and if they have a USB charging port.

I know of one MPPT with USB, but it does seem a common tell.

xcept that's not "what we are working with" as a new panel purchase is required,

A used 175W panel is a ~$20 purchase - not a big deal.

and it must be consistent with the others in the string.

In what way is a 175W panel "not consistent" with three other 175W panels?

How often to MPPT operate at low power in low light conditions? 100% of the time.

I think you are arguing against yourself here. In "low light", my 100V PV's will be putting out (say) 30V which will be in the "higher efficiency" part of the curve.

54.9V/24.07A = 4.38% loss with 10awg
78V/22.76A = 4.63% loss with "much cheaper" 12awg (your argument).

54.9V * 24.07A = 1321.44 W
78V * 22.76A = 1775.28 W

Your maths says the % loss is the same when you use cheaper cable, but run higher voltage and higher power.
(I don't disagree, but .... you said it)

You provide 17 and 34V data, but you are proposing ~68V.

1. I didn't make the graph. I just took what was available in the manual.
2. As the light drops, so does the PVout (or delta-V), which improves the efficiency (your argument).
3. I thought I was proposing 80V, not 68V?
4. the graph I showed is irrelevant as correlates efficiency against output load under constant input (seriously, who cares what the efficiency is when you have full sun and your load is only 20%)

Bored now - moving on.