Hi Basil, thanks for your report.

>>>>> "Basil L. Contovounesios" <contovob@tcd.ie> writes:
>>> AFAICT, this is because TeX-run-format uses TeX-TeX-sentinel, which
>>> calls TeX-TeX-sentinel-check, which expects TeX-error-report-switches to
>>> have been initialised by TeX-run-TeX, but we're not using TeX-run-TeX!

I agree with your analysis. I made a tentative fix as attachment, which
moves initialization of `TeX-error-report-switches' into
`TeX-format-run'. Could you test whether it works on your side?

> The bug report is about the inconsistency between TeX-run-format and its
> sentinel TeX-TeX-sentinel.  In some cases the latter expects
> TeX-error-report-switches to be initialised, but TeX-run-format never
> does this.  IMO if TeX-TeX-sentinel is going to be used with
> TeX-run-format then it should not assume that its process was started by
> TeX-run-TeX.  Unless I'm missing something?

I think this is a AUCTeX bug. `TeX-run-format' is a valid choice for
"How" item of `TeX-command-list' according to its defcustom. However,
current implementation of `TeX-run-format' doesn't achieve its job as
your analysis shows.

>>> I'm not sure whether the TeX-error-report-switches initialisation should
>>> be moved to TeX-run-format, or TeX-TeX-sentinel-check should guard
>>> against an uninitialised TeX-error-report-switches.

Actually, it isn't clear, at least for me, which is the proper fix. :-)
It's very difficult to determine what the proper behavior of
`TeX-run-format' and `TeX-run-TeX' should be, from the relevant doc
stirngs:
,----
| (defun TeX-run-format (name command file)
|   "Create a process for NAME using COMMAND to format FILE with TeX."
`----
,----
| (defun TeX-run-TeX (name command file)
|   "Create a process for NAME using COMMAND to format FILE with TeX."
`----
,----
| (defcustom TeX-command-list
|   [...]
| TeX-run-format: As `TeX-run-command', but assume the output is created
| by a TeX macro package.  Return the process object.
| 
| TeX-run-TeX: For TeX output.
`----
Actually (again), not all the TeX-run-* series are maintained well. The
ones which are seldom used sometimes have nonsense implementation:
https://lists.gnu.org/archive/html/auctex-devel/2017-05/msg00014.html

Regards,
Ikumi Keita