I'm not sure if one could use 'obtainable,' but regardless, such a reading ignores the context. There are two very good understandings of this passage.
One is that the Greek word used for robbery can also mean "something to be forcibly retained or held on to," or so it is said. This would imply that Jesus was God but, for the sake of the salvation of man and the redemption of creation, he would not forcibly or tightly hold on to his privileges as God.
The second meaning, which at first doesn't imply that he is God, is that he did not see equality with God as something to be "snatched violently," or "seized." In this sense, it doesn't mean that he isn't God, but rather that in his humbled state as a man, as a servant, he knew men would be see him as being equal with God through his death and resurrection, rather than compelling men to worship him by somehow using his divine nature. He did it through his suffering and humiliation rather than This is further supported by verses 9-11.
Looking at the context, first notice that Christ emptied himself. He did the emptying. Second, notice where this statement lies: directly in between the two statements "counted it not a prize to be on an equality with God," and "taking the form of a servant." It is very important to keep this in mind. This leads to a significant point that you have missed:
He emptied himself. That is, he was full of something of which it was necessary to empty himself of in order to become a man. It is very significant that Paul uses this phrase to transition from speaking of Christ being in the "form" or "nature" of God to being in the "form of a servant."
The verses from Isa. are statements of monotheism and neither prove the doctrine of the Trinity false nor true.
