I think the two, beliefs and purpose, must be in total sync or the whole will get lost, as is the case today for many areas in MJism, including here on CF, in my view.
I understand what you are saying Avodat. But I am only speaking from my experience. Which experience might not reflect the experience of others.
What I have seen of the movement is the purpose changing, then the teaching changing to agree with the purpose.
But first let me back up a bit.
Please keep in mind when I speak of MJ, I speak to the "shift" reflected in the name change from Hebrew Christian, to Messianic Jew/ish.
The difference seems to be the movement (shift) was founded or based upon "events", not so much theological belief. It was mainly "event" driven.
These events eventually lead to theological shifts.
1st event
Large influx of Jew's coming to faith.
Purpose: to create communities to accomodate Jewish believers to feel more comfortable and to aid in evangelization of Jew's.
2nd event
Rabbinic Jewish community not favorable. Bring legitimate criticism upon Messianics
Purpose: Shift to more strict agreement to Rabbinic requirements.
3rd event (Could be simultaneous to the second event not sure).
Large influx of Gentiles to Messianic congregations. With all their various ideas of why Messianic was for them. Many, having to do with disagreement with "whatever churches" they were against.
These "events" Are what shifted both the purpose as well as their theologies to respond to these events.
Theology never was the driving force for Mjism in the beginning. Doctrine began to shift depending upon events, much of which was not foreseen, and unintended.
Other groups historically were formed from a theology which differed.
Just a loose over view of things from my experience as an mj.