I've searched the forum but haven't found a clear answer and I am not sure how to do this. I would like to accomplish two things with thickbox:

1. From an album thumbnail page, I'd like thickbox to display the full-size image directly without having to go to the single image page. That is, album>thumbnails>thickbox rather than album>thumbnails>image>thickbox.

2. I would also like to display the comment form (with name, comment, and email fields) in the thickbox popup. A comment thread will not be displayed, I'm just using this as a way for a user to privately contact the artist about a particular image.

Is this possible?

Yes, but not out of the box. You will have to do some work yourself.

1. You have to change the url within the next_album loop to link to the full image. See the theming tutorial and the documentation.
2. Probably you need to use the Thickbox inline content method for this. Or create an extra theme page for this to open with the iFrame or Ajax thickbox method and setup the current image object on that.