The Kadam Emanation Scripture is definitely regarded as the text being held by Lama Losang Tubwang Dorjechang in the field of merits for Lama Chopa sadhana.
It is the source of the uncommon Vajrayana Mahamudra of the Gelug, and practices such as Lama Chopa, Long-life Yamantaka, and White Heruka.
Not visible to ordinary beings. In fact, there is a story where one of the lineage Gurus had a shelf with stacks of pechas and in one of the stacks there appeared to be a hole between two of them. A student asked "what's that" and the Lama replied "the Kadam Emanation Scripture". It was supporting the pechas stacked above it.
As times degenerate further and further we see that the lineage Lamas start revealing more and more the things that were once secret. Geshe Chekawa's 7 point mind training and Lama Chopa are examples of this. Another example is the wide spread granting of Highest Yoga Tantra empowerments without checking the Guru-disciple relationship or whether or not recipients are qualified, and the publication of tantric texts for easy access to anyone.
A couple reasons for this regarding the tantric stuff are that the lineage would burn out if it was kept secret any longer, they want to prolong the existence of BuddhaDharma even though we're at the end of Buddha Shakyamuni's teachings, and they want to make connections and give virtuous imprints for these instructions to as many people as possible, not to mention provide an opportunity for people's imprints to ripen right now for such realizations.
According to the common description as far as procedure is concerned, most of us don't have the proper internal preparation through the common path to receive the tantric initiations or contain the tantric realizations, yet, we have the karma to receive the initiations and gain the realizations non the less. What does that say about our imprints? This should be an inspiration to seize this opportunity. Because the tantric sadhana is the union of Sutra and Tantra, we can gain both the common and uncommon realizations of these in one practice if we're skillful. This is especially the case with Lama Chopa.