What I think is not important ...
What we are discussing here is our understanding of teachings that we received or read so what you think is the only thing that matters actually. My point in this discussion isn't that you have to practice a Yidam only that every single teacher of Dzogchen have done it and keeps doing it even a relatively unusual teachers such as Namkhai Norbu, that actually teach Dzogchen as a stand-alone path, is finding termas belonging to the "lower" yanas (his Mandarava terma for example).
You still have to produce a single teacher that actually discarded the "lower" 8 yanas after starting to practice Dzogchen. Also you have to explain why the 17 Tantras of Dzogchen Mengakde contains so much "Tantric" material.
The OP wanted know if you have to practice Yidam if you want to practice Dzogchen. The answer is of course, as several people noted, that it is impossible to practice Dzogchen without a teacher and that one should ask ones teacher about the particular path you should take. A little more general one could, like I did, also mention that since the main Dzogchen traditions that are available belong to the Nyingthik tradition. They contain a far amount of Ngondro and Keyrim and Dzogrim instructions both as preliminaries to Dzogchen instructions of Trechö and Tögal (and other instructions) as well as main practice.
Still what I thinks is not important ...
If Namkhai Norbu teaches lower yana it's because of the capacities of those who attends his teachings ... and he clearly told it so during one of his teaching, that I had the chance to listen.
I gave you many quotes of Kunjed Gyalpo, and Longchen Rabjam, and others that repeat it, and repeat it ... may be they say so for no reason ?
The cursus of most of those masters is a monastic cursus, therefore they follow a LamRim ... then have been instructed in Dzogchen.
Preliminaries are preliminaries ... as for Dzogchen, it is clearly said (in the Khadro Nyingt'hig teachings) that "In order to realize Dzogpa Chenpo, must of us need to develop through gradual stages of training. This is why the lower yanas are essential stepping stones for the higher ones ..."
Again the citation of a Dzogchenpa may elnlight our understanding, the Third Dodrupchen explains : "Use intrinsic awareness as the way. Maintain only that awareness. Do not employ any concepts, since concepts are mind. Meditate having distinguished between mind and intrinsic awareness".
But honestly, I do not see any reason to continue this discussion, because what ever quotation I can provide, you always come back with your own view, pretending that there could be a cause to Dzogpa Chenpo, and that it would be Mahayoga. I do not say that Mahayoga, is not usefull in the management of the body energies, and that is of course usefull to stabilize rigpa ... but this is not a necessity, because once in rigpa, "energy balance" is set on automatically.
I will end, once more, this time with a quotation of the Gnas lugs mdzod ...
"For the categories of Mind, Expanse, and Direct Transmission,
the consummate meaning of the heart essence
is that of ineffability, openness, spontaneous presence, and oneness"