Although this may not provide that much guidance on meeting structure, I would still recommend http://bahai-library.com/compilation_im ... _deepening
As far as deepening class methods, it does have at least one quote of relevance, indicating:
"By "verities of the Faith" he means the great teachings and fundamentals enshrined in our Bahá'í literature; these we can find by reading the books, studying under Bahá'í scholars at summer schools and in classes, and through the aid of study outlines."
(From a letter dated 19 April 1947 written on behalf of Shoghi Effendi to an individual believer)
I think the use of study outlines as recommended here is a very much under-appreciated tool for focusing study. It can help summarize the guidance and give a skeleton by which people can not only understand the material, but also be able to internalize the underlying logic (or history, etc.) and explain it to others, with or without being able to memorize a lot of supporting details (or to use their own, as 'Abdu'l-Baha did in varying the details of His talks). With an outline, one can realistically ask people to make a presentation themselves on the topic as an outline does half of the work already.
Although an outline may seem better suited for logical talks or history, one can also do this in a sense with topical studies (such as you mention on unity) by creating the likes of Lights-of-Guidance-style headings derived closely from the text so that people would be able to internalize and describe, for example, what some of the implications of unity were.
Fill-in-the-blank type of questions may help some people to focus on the literal text (a lot of people have trouble with even that), but an outline is a lot more engaging, challenging, and in the end more powerful (unless you happen to be someone good at memorizing), and as such, can sometimes engage people who would not otherwise be engaged.
It can also be used as the basis for encouraging talks to be given, e.g., at Feast (where talks are supposed to be encouraged) or at firesides, including especially by youth, and when participants know they may be called to use the material, they should be even more motivated.
If the deepening is not for youth, then one might wish to consider how to bring them in, as this is what the Baha'i Writings ask when referring to deep study of the Writings (for older believers to train younger ones).