Implementing Thumbnails without auto-truncating excerpts (using "more")

I’ve searched a bunch, but I need a little help completing this picture.

Here’s what I want:
A default settable way to have a truncated post on the main page, where the lead image is linked to the full post. I want the truncation point to be defined using the “more” tag/break/feature, not a predefined #### of words/letters/lines.

Currently, we just use the standard WP picture insert for the lead shot, and set the link to “none”, then use “more” to get past the jump.

On the off chance that someone screws up and makes the picture clickable, users who click are taken to the image/attachment page, not the full post.

I believe using thumbnails is the best way to make the image clicky to get to the full post, but it seems it’s only possible to use thumbnails with excerpts enabled, and with excerpts enabled, you have to set them to 55 words or whatever. I want to keep using “more”.

Within the post editor, I see the option to run a “featured image”…is that the same as a thumbnail?

Lastly: is there a way to preview what a post will look like on the main page (while truncated)? The preview always shows the whole thing.

Misc info that’s relevant:
–Site is hooniverse.com (so you can see how we format stuff)
–Using Atahualpa 3.5.3
–I’ve got multiple writers/users, so I need this to work for every WP user level.