My (limited; one-time only) experience with Diamond Way--a bunch of young people get together in a living room, sit down, and play a cassette tape of Ole speaking in English, "leading" what's called, by their centers, the "Three Lights Meditation "-it's actually a Guru Yoga based on, and composed by, 16th Karmapa, Rangjung Rigpai Dorje, (A Shower of Ambrosia Generating the Shoot of the four Kayas, is the actual name: bla.ma'i.rnal.'byor.sku.bzhi'i.myu.gu.skyed.byed.bdud.rtsi'i.char.rgyun.ces.bya.ba.'zhugs.so). It was presented somewhat like a guided meditation. Say some "Karmapa Khyenno" mantras, a couple minutes of silent meditation, and you're done. Everyone gets up, but a few of the guys pass around a page in Tibetan, which they proceed to chant, quickly, standing up (turns out it's what we call the "Ma Dag Ma," a short Solkha to Karma Kagyu Protectors). Lots of laughter when it's finished, then it was social hour. A card table in the living room, with Ole's books on display for purchase.
Comparing it to any other Tibetan Dharma center, including even Vajradhatu/Shambhala centers, it was like apples and oranges. But hey, it was just one visit....can't say it was the typical experience.
Not completely untypical I'd say, although I disagree with your implication that people don't seem to know what they're doing. Every member of DW knows the 16th Karmapa meditation, knows the meaning of value of Guru Yoga, and takes the practice to heart. The meditation is usually led by one of the senior group members. The group repeats the Karmapa Chenno mantra together. The 'silent meditation' you mention is of course the completion phase of the meditation, and the guiding text explains it as well if not better than other Sadhanas.
The invocation of Mahakala is done after every group practice, and most people use it to finish their own meditation session at home as well. Beginner members of the groups often don't know the meaning so well, but those with more experience certainly do. Translations of the text are available. The empowerment for Mahakala has been given a few times by Karmapa Thaye Dorje. Ole often explains it's power, in connecting the DW centres to the lineage, and keeping the protection of that lineage in daily life. It is done sitting, not standing. Those who know the meaning will usually do more than just chant, but actually visualize the practice, and if you received the empowerment, finish with some mantra's and completion.
The group practices are indeed very short. These 'open evenings' are an introduction to meditation, and indeed a social gathering. That makes it easy for new-comers to join, try the practice, connect to people, ask questions in an informal way. And it's just fun! I don't see anything wrong there. Please don't believe that the students don't practice more than this. Everyone has his own personal practice. Usually ngondro followed by the 8th Karmapa Guru Yoga (Tunshi Lamai Naljor). I have not seen any other sangha in which such a great number of people practiced and completed the ngondro.
Besides the Ngondro, Guru yoga of 8th Karmapa, and Mahakala, there is a few other practices:
First of all Chenrezig, which is not practiced a lot in groups anymore, but mostly individually. I'd guess that Ole has told more than 50% of his long-time students at some point to do this practice. Often 100.000 mantras. And if you tell him you finished, another 1.000.000
(and so on, I'd guess, until the result is there)
Clear light, which is a practice involving yogini, given by the 15th Karmapa, to remain in awareness during sleep. It's very concise (in fact a super stripped down version of one of the yogas), and works wonders.
Phowa, which is given in big courses by lama Ole. The practice of conscious dying. Very powerful. No matter what your believes about Ole and the DW are, he is a real master of this practice, and has benefited countless people by teaching this practice, and helping them live and die in positive ways. To paraphrase Ole, 'death to a yogi, is like waking up a little'.
Some side practices, which are not done all that much, like Tsepame, Sangye Menla, etc. Indeed deities are often referred to in their English names, 'Buddha of Long Life', 'Medicine Buddha', etc. Ole often gives a simple practice for Medicine Buddha, where people not only learn to practice it themselves, but also learn how to invoke it for others who are ill.
Finally, Ole sometimes gives a practice at big courses, which I won't mention here. Think of it as a combination of great blessing, and a pointing out instruction. That's to find out for those who go in a little deeper, this would not be the place to discuss it.
So yes my friends, the practices in the DW centres are very simple. You can learn it all, without ever having to make tormas, do mudras, buy a dorje, bell, serkhym etc. Just the essence. People practice hard, and get results. Indeed you might say that the knowledge of the tradition is limited within DW groups. But it's certainly an authentic part of the tradition. Again, no matter what you think of Ole as a person, I think he created a beautiful concise format for practicing the essence of Vajrayana in a modern style.
(Full disclosure: I've been a DW student from 2002-2007, but now follow Chokyi Nyima Rinpoche in search of more depth and the connection to certain teachings. To his credit, Ole supported me in making that step. I still visit his centers every now and then. Any questions about this matter, public or private, are welcome)