It is still enormously difficult to understand what I am supposed to
do! You really ought to test out your system on a set of users who are
not familiar with it and are not web design experts, see what they have
trouble understanding, and do something about each such problem; e.g.,
put links into the pop-ups users get on using Siteimprove on a web
page, linking to pages they can go to which will tell them what
to do, with examples they can imitate

E.g., when I click on the Siteimprove tool for many of my pages, one
of the complaints is that the "language" is not specified. Fortunately,
I found that my home page, which was designed by specialists decades
ago before I knew anything about html, did not produce this complaint,
and I found that it had a line . I guessed
correctly that lang="en" specifies the language. I wasn't sure what
dir="ltr" means, but I checked online and found that out; so now
I know to put this on all my pages that don't have it. But your
Siteimprove tool ought to give, with the diagnosis about language,
a link that the user can click on and be informed of this immediately!
(Maybe it does -- but if so, it's not clear where it is.)

Incidentally, in addition to testing your system out on naive users,
it might also be good to take a few of your workers who know the
system from top to bottom, and ask them to put themselves in the
place of someone naively reading the instructions, and try to note
things that they might not understand. They might know subtleties
of the system that naive user might incorrectly think they understood,
but that they actually misunderstood. (But if it's a choice between
checking it on naive users and on experts, the former seems most
important.)

A year ago, when these accessibility requirements were first brought
to our attention, I received a long spreadsheet of problems on pages
on my website. This suffered from the above difficulties, but at
least it was presumably complete. This time, I don't see any way
to get such a list -- do I have to go to each of the many many
pages on my website and apply the Siteimprove tool to them one by one?

The addon ask to create an account but its optional.

works great! it highlight the problematic html element and give explanations and how to fix!
the Exporer tab let you see your website with different colorblind filters.

Very unclear what I have to do to sign in; whether what I have done did it.

It is very helpful to see the critical problems for the accessibiliy and to get some advices how to solve them.

Easy to use, accurate, and reliable. A must have for content contributes, web admins, and developers.