1. I get output (as "Reprint") for chicago-fullnote-bibliography.csl. Does that not work with pandoc? Have you tried with Zotero?
2. As Nick points out later in that discussion, CSL doesn't distinguish between ca. and uncertain (and neither does Chicago Manual) so we can't fix this within current CSL and it's unclear if it needs fixing at all.
3. We can fix.

On point 1 (original date), yes, my apologies, I see "repr. ...".

<li id="fn1" role="doc-endnote"><p>Marcus Aurelius, <em>Meditations</em>, ed. Maxwell Staniforth (161AD–180AD; repr., London: Penguin Books, 2005).<a href="#fnref1" class="footnote-back" role="doc-backlink">↩︎</a></p></li>

On point 2 (circa) I don't think it is working for pandoc and chicago-fullnote-bibliography.csl.

---
author: Joseph Reagle 
title: "Test"
suppress-bibliography: true
...

test of [@Aurelius2005mns]

---
references:
- id: Aurelius2005mns
  type: book
  publisher-place: "London"
  author:
  - family: "Aurelius"
    given: "Marcus"
  issued:
  - year: 2005
  original-date:
    - year: 180
      circa: true
  editor:
  - family: "Staniforth"
    given: "Maxwell"
  ISBN: "9780143036272"
  publisher: "Penguin Books"
  title: "Meditations"
...

yields

╰─➤  pandoc -f markdown+yaml_metadata_block -w html --csl=chicago-fullnote-bibliography.csl --filter pandoc-citeproc test.md
<p>test of<span class="citation" data-cites="Aurelius2005mns"><a href="#fn1" class="footnote-ref" id="fnref1" role="doc-noteref"><sup>1</sup></a></span></p>
<section class="footnotes" role="doc-endnotes">
<hr />
<ol>
<li id="fn1" role="doc-endnote"><p>Marcus Aurelius, <em>Meditations</em>, ed. Maxwell Staniforth (180AD; repr., London: Penguin Books, 2005).<a href="#fnref1" class="footnote-back" role="doc-backlink">↩︎</a></p></li>
</ol>
</section>

It just says "180AD" without any indication of circa...

oh, that's circa on the original date. That's indeed not in the style. We can try to add that, too. No guarantee on how quick either of those are going to happen, but obviously happy to take PRs from you or anyone else.

I've been searching the online CMS 17 and it only recognizes a "guessed at" date, with two different ways of representing it.

A guessed-at date may either be substituted (in brackets) or added. See also 14.132.

Boston, n.d.
Edinburgh, [1750?] or Edinburgh, n.d., ca. 1750

@adam3smith can you recommend an existing style that does circa on an old date? Looking around, it looks like this CSL:

<choose>
      <if is-uncertain-date="original-date">
        <text term="circa" form="short" suffix=" "/>
      </if>
</choose>

pick up this type of yaml?

  original-date:
    - year: 180
      circa: true