How to handle blog sub-categories with Runway?

Hi there, I'm almost finished reconfiguring my blog to Runway.

I'm coming up against some issues with my archive of sub-categories.

In my post.html template, I display the parent and sub-cat for a given post, as follows:

<perch:categories id="categories" set="blog" label="Categories" display-as="checkboxes">
            <a href="/blog/category/<perch:category id="catSlug" type="slug" />" class="p-category">
                <perch:category id="catTitle" type="text" suppress="true" />
            </a>
</perch:categories>

And in post_category_list.html:

<perch:before>
<ul>
</perch:before>
    <li><a href="/blog/category/<perch:category id="catSlug" />"><perch:category id="catTitle" /></a></li>
<perch:after>
</ul>
</perch:after>

I've also added the following patterns to my archive page:

blog/category/[slug:cat]
blog/tag/[slug:tag]

When I hover over the parent category, it links to blog/category/[parent] and the archive displays the posts in that category. All good.

When I hover over the sub-category, it links to blog/category/[sub-cat], but archive displays nothing.

This is the debug for the blank sub-cat archive:

Time    Δ   Debug Message
0.0812  0   SELECT p.pagePath, pr.routePattern, pr.routeRegExp, p.pageTemplate FROM perch2_pages p LEFT JOIN perch2_page_routes pr ON p.pageID=pr.pageID ORDER BY pr.routeOrder ASC, p.pagePath ASC
0.0823  0.0011  Matched route: blog/category/[slug:cat]
0.0824  0.0001  Using master page: /templates/pages/blog/archive.php
0.0824  0   Page arguments:
Array
(
    [0] => /blog/category/exteriors
    [cat] => exteriors
    [1] => exteriors
)
0.0844  0.002   SELECT u.*, r.* FROM perch2_users u, perch2_user_roles r WHERE u.roleID=r.roleID AND u.userEnabled=1 AND u.userID=1 AND u.userHash='0b639f8e7c89de87d9d4aa367cc169a9' LIMIT 1
0.0849  0.0005  UPDATE perch2_users SET userHash='818be44017dd764b75de6ce98f32cf0c' WHERE userID='1'
0.0852  0.0003  SELECT p.privKey FROM perch2_user_privileges p
0.0941  0.0089  SELECT * FROM perch2_pages WHERE pagePath='/blog/archive' LIMIT 1
0.0983  0.0042  SELECT DISTINCT settingID, settingValue FROM perch2_settings WHERE userID=0
0.1052  0.007   SELECT setID FROM perch2_category_sets WHERE setSlug='' LIMIT 1
0.1056  0.0003  SELECT main.* FROM perch2_categories main WHERE 1=1 AND (catPath='exteriors/') ORDER BY catTreePosition ASC
0.1059  0.0003  Using template: /templates/categories/blog_cat_archive.html
0.1097  0.0038  Using template: /templates/mailchimp/subscribe.offcanvas.html
0.1152  0.0055  SELECT setID FROM perch2_category_sets WHERE setSlug='' LIMIT 1
0.1168  0.0016  SELECT main.* FROM perch2_categories main WHERE 1=1 AND (catPath='exteriors/') ORDER BY catTreePosition ASC
0.1173  0.0006  Using template: /templates/categories/blog_cat_archive.html
0.1178  0.0005  SELECT DISTINCT idx.itemID FROM perch2_blog_index idx JOIN perch2_blog_posts main ON idx.itemID=main.postID AND idx.itemKey='postID' AND (idx.indexKey='_category' AND idx.indexValue LIKE 'blog/exteriors/%' OR idx.indexKey='_category' AND idx.indexValue='blog/exteriors/')
0.1192  0.0015  SELECT SQL_CALC_FOUND_ROWS DISTINCT tbl.* FROM ( SELECT idx.itemID, main.*, idx2.indexValue as sortval FROM perch2_blog_index idx JOIN perch2_blog_posts main ON idx.itemID=main.postID AND idx.itemKey='postID' JOIN perch2_blog_index idx2 ON idx.itemID=idx2.itemID AND idx.itemKey='postID' AND idx2.indexKey='postDateTime' AND idx.itemID IN (NULL) WHERE 1=1 AND idx.itemID=idx2.itemID AND idx.itemKey=idx2.itemKey GROUP BY idx.itemID ) as tbl WHERE (postStatus='Published' AND postDateTime<='2015-11-30 18:07:00' ) GROUP BY itemID ORDER BY sortval DESC LIMIT 0, 4
0.123   0.0037  SELECT FOUND_ROWS() AS `count`
0.1329  0.0099  Using template: /templates/blog/archive.html
0.1339  0.001   SELECT t.tagTitle, t.tagSlug, COUNT(p2t.postID) AS qty FROM perch2_blog_tags t, perch2_blog_posts_to_tags p2t, perch2_blog_posts p WHERE p2t.tagID=t.tagID AND p2t.postID=p.postID AND p.postStatus='Published' AND p.postDateTime<='2015-11-30 18:07:00' GROUP BY t.tagID ORDER BY t.tagTitle ASC
0.1358  0.0019  Using template: /templates/blog/tag_link.html
0.1476  0.0118  Using template: /templates/mailchimp/subscribe.html
0.1506  0.0031  Time: 0.1506
0.1507  0   Memory: 5.529

I've played around with changing catSlug to catPath in some places (template links as well as creating a ~~~blog/category/[slug:catPath]~~~ token (???), but a new problem arises...catPath turns URLs into: ~~~blog/category/blog/[parent]/[subcat]~~~ ...which is no good.

Are you guys able to shine any light on this? Thanks very much!