On January 21, 2023 I contacted the Pastors at Tsem Rinpoche's website under the "Ask the Pastors" site.
I asked them about Tsem Tulku making Dorje Shugden a YIDAM and here is their answer.
QUOTE
Reply by pastor
Dear M,
Tsem Rinpoche assigned people YIDAMS based on their needs and which practices would be most beneficial for them. Pastor David was referring to Rinpoche assigning Dorje Shugden for people as a YIDAM, meaning the deity that they should rely on the most and focus on, to be their main practice.
There is no practice or text of using Dorje Shugden as a proper yidam, meaning tantric practices of self-generation, etc. When Rinpoche assigned Dorje Shugden to be their YIDAM, this was in reference to which deity they should focus on in their practice.
The practice they follow is no different from the standard form of Dorje Shugden practice as a Dharma Protector, it is that they focus and rely on Dorje Shugden more than any other deity. There is no sadhana or practice of Dorje Shugden as an actual tantric deity, only as a Dharma Protector. As an emanation of Manjushri, Dorje Shugden’s practice can be your main practice, though following the standard formula as a Dharma Protector and not a tantric deity. Thanks.
UNQUOTE
There you have it, Dorje Shugden can be your main YIDAM practice so say the Pastors of Tsem Tulku.
Thank your Rinpoche for the courage to make Dorje Shugden a YIDAM.
Pray for the swift return of Tsem Tulku.