"Einstein considered that the relativity principle was not compatible with a preferred frame. "
You are stooging yourself by your unwillingness to speak in plain English.
The principle of relativity is consistent with an integrated holistic reality. It is incompatible with a theory of velocity absolutism.
GMB, I have no dog in this fight, but will ask you how sol's and others' use of vocabulary, and your's, are any different in being possible obstacles to understanding?
I've been long since fed up with overuse of buzz words and jargon, and prefer plain speaking.
Please help out this audience member by adapting the manner of plain speaking that you insist upon from others. It will help me follow the discussion, though I admit to a nasty habit: when I see words and terms I don't understand, I either look them up or ask to have them explained in simpler English.
So, in simpler English, what is:
integrated holistic reality.
theory of velocity absolutism.
I gather that the latter, from contextual clues you have offered, is your description of the use of constants, in this case speed of light as a constant.
The former I need your help with.
GMB said:
Yes velocity is relative. And contains no acceleration in it so no g forces or what have you.
Well velocity IS relative.
Velocity at a given moment in time while under acceleration can be easily calculated by the equation
v = at
Let's say my helicopter is under acceleration for a minute as I go from zero to 120 knots. It has an
a and a variety of values of
v for that whole minute. I continue applying acceleration
a to counter the
-a of drag from air resistance, since air is needed for lift and thrust. I thus maintain a constant velocity of 120 knots in a given direction, let's say North. (I want to visit Mrs Claus whilst Santa is out.

Nudge Nudge, wink, wink, say no more.)
GMB, I may be playing with words here, but let me suggest to you the following:
My helicopter's velocity of 120 knots contains in it, as you say it, an acceleration
a necessary to overcome -a (drag) of the air. We then sum accelerations to zero, leaving velocity constant until I either increase or decrease
a, -a increases or decreases for environmental reasons, or the usual happens with helicopters and I land just before I run out of gas.
But special relativity is a theory of velocity-absolutism. It is in contradiction to the principle that velocity is relative.
Is there no room at the inn for them both? I think that's where you are coming from. Am I close?
DR