Carlos,
There are three important concepts in your question. They are:
The Absolute
Spirit
Matter
These three concepts are known by several names. Here is a short list of similar names.
(1) The Absolute
-Space refers to the Absolute. It is called "Universal Space" in Stanza i-1-8 and "Dark Space" in Stanza i-3-7. (Please note that the phrase "Bright Space" in Stanza i-3-7 refers to the Universal Mind and not to the Absolute.)
- That, SD vol I p. 7
(2) Spirit
-Purusha
-Father
(3) Matter (Mulaprakritic Matter)
- Eternal Parent, Stanza i-1-1,
Man, the Measure of All Things, p. 46 by Prem & Ashish
-Mulaprakriti
-Mother
~~~
You must keep track of the different names for the three concepts, and it is a difficult job.
You quoted,
"THAT" (The Eternal parent, Space) is the causeless cause of both Spirit
and matter. --> That = Eternal Parent = Mother = Matter. In the sentence above, the word "matter" refers to atomic matter, not Mulaprakritic Matter.
The use of the word "matter" can be confusing. It is used with two different meanings in different parts of the Stanzas of Dzyan. Here are the two meanings.
* original matter
* atomic matter
Original matter (also called Pre-matter or Mulaprakriti) is the basic substance of the universe. It is the substance from which atoms are made. Mulaprakriti (original matter) is "eternal", in that it lasts even during periods of cosmic rest.
Atomic matter is Mulaprakriti that has been formed into atoms. These atoms only appear periodically (during periods of cosmic activity), and disappear (devolve back into Mulaprakriti) during periods of cosmic rest.
Please note the word Matter usually refers to Mulaprakriti (original matter) not atomic matter in The Secret Doctrine.
"Matter is eternal, becoming atomic (its aspect) only periodically." (SD vol 1 p 552)
"But, at the same time, HPB says:
The spiritual essence of matter (MATTER) is co-eternal and even one with Space (THAT, The Eternal parent) in its abstract sense."--> The spiritual essence of matter is MATTER. Restated, The spiritual essence of atomic matter is Mulaprakritic Matter.
"So, looking at these declarations I note a clear contradiction as "The Eternal Parent" can not be the CAUSE of both Spirit and Matter as HPB said."--> The Eternal Parent is Mulaprakritic Matter. There is one more aspect of Mulaprakritic Matter which needs to be mentioned. Mulaprakritic Matter goes through a change when the first differentiation of the new universe begins. Blavatsky refers to this change as the transformation of Mother into Father-Mother. At the beginning of a Manvantara, Father and Mother link up to form Father-Mother. Let's take a closet look at Father-Mother.
"... the gods still sleep in the bosom of 'Father-Mother'. "(SD vol I p. 109)
"... evolved from the ONE Element in its second stage (Father-Mother)...." (SD vol I p. 140)
"... into the depths of Father-Mother AEther." (SD vol I p. 564)
" 'Darkness is Father-Mother: light their son,' says an old Eastern proverb." (SD vol I p. 40)
"Thus Adam Kadmon and Adam-Adami came to mean: 'The first emanation of the Father-Mother or divine nature,' " (SD vol II p. 43)
The nature of Space changes with the beginning of a Manvantara.
"Space is called the 'Mother' before its Cosmic activity, and Father-Mother at the first stage of re-awakening. (See Comments, Stanza II.)" (SD vol I p. 18)
"In the beginning, before Mother became Father-Mother...." (SD vol I p. 74)
"The Breath of the Father-Mother issues cold and radiant and gets hot and corrupt, to cool once more, and be purified in the eternal bosom of inner Space...." (SD vol I p. 144)
Let¡¯s review.
The Eternal Parent = Mother = Mulaprakritic Matter
before it became Father-Mother.
Matter = Mulaprakritic Matter
after it became Father-Mother.
"as HPB said¡
Matter is CO-ETERNAL and even one with Space (The Eternal Parent), so it can not be a creation or an EFFECT of the Space (The Eternal Parent)!"--> Matter has three different definitions (three different forms).
(1) The Eternal Parent = Mother = Mulaprakritic Matter before it became Father-Mother.
(2) Matter = Mulaprakritic Matter after it became Father-Mother.
(3) Atomic matter. Atomic matter appears when ¡°holes¡± are dug in Mulaprakriti.
"The vast sphere in space, within which the sun and its planets were to arise, contained at the beginning no substance in any way akin to matter (visible or invisible) which we have within the system today. There was only Mulaprakrti or "root-matter", that ether of space once postulated by science which is incomprehensible to our imagination, since it is only out of "holes in the aether" that matter such as we know is composed. In our Theosophical studies we have called this primordial ether or negation of matter "Koilon", the "emptiness". (Fig. 64.)

"Into this Koilon, or primordial ether of "space, the Cosmic Logos poured His energy, pressing back the Koilon from innumerable points within it. (Fig. 65.)

"Each "bubble" or point of light exists where Koilon is not; therefore each bubble is in reality a point of consciousness of the Third Aspect of the Cosmic Logos; and each bubble persists only so long as He wills to keep back the enveloping Koilon."
(First Principles of Theosophy by C. Jinarajadasa)
http://users.ez2.net/nick29/theosophy/fp-192.htm~~~
Therefore, Blavatksy defines the word matter three different ways, and she mixes up the three definitions in her writings with gusto. If you can keep track of the three different definitions, The Secret Doctrine will start making sense.
Carlos, these are advanced studes you are undertaking, and I congratulate you for making the effort. I have written a study guide on the Stanzas of Dzyan.
http://users.ez2.net/nick29/theosophy/stanzas.htmRlease read my study guide and let me know if it helps.