Ah, you are slowly seeing the limits of WordPress, and you'll understand why my support of WP only amounts to "tolerance" and not often to "praise". MODX handles this sort of customizations *much* more easily.
Yes, the Summarize Posts plugin can be used to help pull this off. CCTM 0.9.5 will integrate the Summarize Posts plugin, and I've put in a number of improvements there, but seeing as it's the holidays and I'm stumped with a couple of the integration points, 0.9.5 won't be publicly released for a couple more weeks.
You mention that you want a "fair bit of text" at the top of the archive pages: do you mean you need to add changing content to that page? That's difficult because it's a dynamically generated page, so that would land you in customization-land because WordPress doesn't let you manage content inside the archive.php page(s)... you can only edit the template and it's HTML directly.
So here's what you could do as a work-around: create a dedicated page for displaying archives. That way you can edit the content of that page in the manager just like any other page or post. You can put the code in that page's template that pulls up the requested year and month, but in order for that to work, you'd have to basically re-write the code that generates the archive links. It gets complicated quickly because you have to refactor a lot of built-in WP functions (icky).
I have used this plugin before:
http://kwebble.com/blog/2007_08_15/archives_for_a_category
That lets you generate archive links filtered on a category, e.g.
<?php wp_get_archives('cat=5'); ?>
Hope that helps -- there's not really an easy solution for working around the WP architecture unfortunately.