Taking up the invitation to clarify a few points from a Theravada point of view...
So in Theravāda buddhahood is known
Yes, to the one who rediscovers the Dhamma and instigates a "sasana" (tradition, dispensation etc.). We are still living in the current dispensation (and have been for 2600ish years) and not until this current dispensation has ended through the Dhamma being lost, and being later rediscovered (by Metteyya, aka Maitreya) will there be another Buddha in this world-system. Until such time, the goal is as the Buddha taught in the Sutta Pitaka, namely arahantship.
In Theravāda the student works towards having that flash of insight and in dzogchen the master directly introduces it immediately.
A bit vague... there's lots of different ways of going about things in Theravāda. If you're suggesting people are shielded from Right View and are told to go and sit and do some particular technique in the hope of cultivating a "flash of insight" which the teacher can later translate... well, there are one or two meditation subcultures with Theravada that do this, but it's a recent development and certainly not the traditional way of going about it. On the flipside, there's even some subcultures that teach only Right View, and do not teach any formal meditation. As for withholding anything, the Buddha of the Sutta Pitaka did not teach with a closed fist.
Another thing being that Theravāda clearly has aspects of renunciation, in that certain qualities and aspects of experience are seen as obstacles and are avoided. Dzogchen integrates everything without establishing a duality between good/bad, right/wrong (when it comes to the essential view). Dzogchen only differentiates between ignorance (Avidyā/ma-rigpa) and wisdom (Vidyā/rigpa).
In Theravada, sense-restraint and such is of pragmatic value as opposed to absolute, as it does not establish certain inputs as being inherently one way or another. It is from the contact of sense base and sense that fetters may arise, so sense-restraint is just a pragmatic means to strengthen one's practice - in time no input should give rise to any fetter.
My understanding is Sri Lankan Theravadist do not even recognize the heart sutra as a root text.
You'll have to forgive me that I can't be more pro-active in giving answers, as I'm not familiar with the ways and means of Dzogchen... I can only tell you if the things you talk about have some parallel in Theravada.
Live in concord, with mutual appreciation, without disputing, blending like milk and water, viewing each other with kindly eyesDhamma Wheel (Theravada forum)
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