Sorry but i couldn't exactly get the scenario...
first of all, what WP and Combo Slideshow versions are you running?
are you trying to insert a "custom" slideshow (single slides set in the "manage slides" admin panel) or your "blog summary" (displaying each post's thumbnail)?
Unfortunately, no more than one "custom" gallery are available at the moment.
Shortcodes are obviously meant to be used in the post content and slide-show the post attachments gallery OR the "custom" one.
In order to get it in your homepage you have to manually insert the manual code in the template files, as you almost did...
but you have to replace
$output = true, $post_id = null because these are only the default parameters and their default values: so it's
$CMBSLD_Gallery -> slideshow(true, '22'); to show the attachment gallery for post ID 22
$CMBSLD_Gallery -> slideshow(true, null, null, null, true); for the custom slideshow
(sorry can't remember if quotation marks are required around IDs or if you can safely use '' instead of null)
But the global blog summary still needs another function
Captions and links should be taken from:
-the post title/link (with global blog summary slideshow)
-the attachment title/link (with post gallery slideshow)
-the slide title/link as set in "manage slides" (with custom)
I'm working on a new version, I'm trying to double-check everything to catch any possible bug, but if you can't wait and want to edit the crucial file gallery.php, I guess I need to better focus on what you're trying to accomplish.