Shakyamuni was Sambhogkaya, Nirmanakaya and Dharmakaya, and their natural unity as Svabhikakaya, when he lived on Earth, it is a mistake to think that Sambhogakaya etc appeared only later.
Actually, what we see on earth in the form of a physical Buddha (if we live in the time of Shakyamuni) is the nirmanakaya aspect. Only to tenth-level Bodhisattvas does the Buddha display the Sambhogakaya.
Have You heard of the chinese master Zhi-yi? How he lectured on the first character in the name of the Lotus of th True Law sutra for 90 days? The existing sravakayana sutras are quite brief, the example of ZhiYi gives us some clue of how vast teachings there have been in the early period of Dharma. I have had an experience that Shakyamuni had taught a teaching of a graded path that contains 100 stages, and he had taught it several times, but nothing of it remains; Avatamsaka Sutra describes a path that has 52 stages, it is the closest thing that we have.
I have certainly heard of Zhiyi, but it doesn't prove anything with regards to the question if the Mahayana teachings were taught by the nirmanakaya. Your visionary experiences (if that is what you meant), admirable though they are, also does not prove anything with regards to what teachings were taught during the Buddha's ministry during his earthly time on earth.
There is a whole chapter of buddhist history that remains undisclosed, namely that most of the early monks and even many Arhats backslided, many Arhats even gave up the whole thing after a few years. These kinds of evolutionary events made it necessary for the Buddha to teach the Prajña Paramita sutras, then later the Tathagatagarbha sutras, the Amitabha sutras, and the White Lotus sutra.
Sarvastivada Abhidharma has a classification of Arhats, that has Arhats that backslide and Arhats who don't backslide. The path is very long, difficult, or endless even. After enlightenment there still remain some really serious problems, when you encounter them you will know what the clues are. Before it you will probably believe the public edifice of the Sravakayana, which not really true at all. This situation is revealed, or merely hinted at, in the Lotus of the True Law sutra.
The Lotus Sutra presents a view that is not always shared by other Mahayana sutras. For example, the Prajnaparamitas do not assert that Arhats and Prateyakabuddhas have to proceed on to Buddhahood, whereas the Lotus insists that they will have to.
It is quite an exercise to imagine that just because some schools consider Arhats may be susceptible to backsliding (and not all schools assert this), then a group of Arhats would purposely block part of the Buddha's teachings. There are no evidence on the ground that even marginally support your assertions.
Sorry - I cannot agree with your theory, since it lacks support in so many areas.