Differentiating between rights as to which is the most fundamental shouldn't be a confusing issue - mostly that is just seeing which has to exist to make possible all others. (Her words were, "...all others are consequences or corollaries...")
You said, "Since saying that a right is the source of all rights and saying that it is the most fundamental right are the same thing..." Not really, although close in this case. "Fundamental can be taken as more closely aligned with importance, and "source" is from whence it came. I came from my mother, I came from Wyoming (depending upon our context), yet while those may address my source, they don't discuss the fundamental aspect of my nature.
And, "...anyone in their right mind would take "the right to his own life" to mean "the right to choose one's actions freely", or some equivalent thereof. However, the "right to life" (both according to Rand's very definition of that right, and according to the fact that we are talking about the right to a process there (i.e. to the process of life), means the right to engage in self-sustaining behavior. Clearly these are two different things entirely."
At this point, I'm not entirely clear on the distinction you are making. But I'll go on as if I did :-)
p.s., As soon as someones says, "anyone in his right mind..." it is like highlighting the following phrases as "here is an argument I don't feel confident to stand alone without trying to scare people away from challenging it by saying they would be crazy to disagree" :-)
"the right to his own life" is more than right to choose one's actions freely. It is all of the actions implied with ownership - including right to dispose of. It is sovereignty. It is a the bedrock of declaring that man is an end in himself and not a means to the ends of others, hence the translation from what man needs to live to what he has the rights to.
The naked, biological process of life - metabolic processes, breathing, moving food from hand to mouth were not what Rand meant as man's life. She talked about life proper to man. That derives from man's nature. Our nature isn't that of a plant or of someone in a heart-lung machine or as someone part of a mindless hive-like community engaged in group-think. And she was clear that right meant freedom to act and not a positive right (a right to receive something for someone else).
Think of two stages in the derivation of rights. First what are the conditions of existence that man's survival requires (given his nature) - a metaphysical exercise detailing facts. Second what are the moral principles that we derive from those conditions and our nature - the building upon metaphysics to establish ethics. Ethics cannot pre-exist life, and ethics should not be conjured into being as a floating abstraction. In the end, we are looking at the same entity, but from two perspectives. (some of this is my take after reading Nicholas Dykes' paper)
Rand always stayed cognizant of context, and she also always stayed cognizant of the layers, the hierarchy of knowledge. I have the sense that you are staying very linear and analytic at a word level at a time when a better start is to see that bigger picture of the hierarchy, then start picking at the words in the that context. But I may just be missing what you are saying.
Did your point get addressed? Or, missed?