To begin with, the word translated here as “nature” in Phil 2:6 is μορφῇ which means “shape”, not “nature”. If Paul had wanted to mean “nature” there are plenty of Greek words that could have been used that give that meaning unambiguously. Exactly what he meant by μορφῇ is not completely clear, but the word refers to outward appearance rather than inner essence and seems to connect to the later reference to Jesus taking on “human likeness”. Since Paul believed in a heavenly pre-existence for Jesus, this probably refers to Jesus abandoning a celestial form and taking on an earthly one. Far less ambiguous, however, is the word translated here rather awkwardly by the phrase “to be used to his own advantage”. The word is ἁρπαγμὸν and a comparison of translations shows its translation varies widely – everything from “a thing to be grasped” to “something to be retained”. But the meaning of the word is quite clear – it means “something to be seized, stolen, looted” and comes from a noun meaning “booty, plunder, something taken in war”. The reasons for the widely varied translations of this word is not that it’s unclear, but rather that its clear meaning doesn’t fit with Christian theology. If Jesus is being depicted as not regarding equality with God as something to be seized, taken or plundered by force, then this means he doesn’t already have it. So the varied translations are working hard to try to make a word that depicts Jesus as not being equal with God into something that means precisely the opposite. Far from proving this means Paul did regard Jesus as divine and equal with God, it actually shows exactly the opposite idea.
First of all, I will concede that there is a range of possible interpretations of Philippians 2:6-9. There are many passages in the Bible for which there is a fairly wide range of possible interpretation. I will discuss the passage in question in a minute, but you suggest that this is the only passage or the most convincing one in Paul’s writings that imply the deity of Jesus. This is certainly not the case!!! There is Colossians 1:15-20 where Paul tells us that Jesus is the firstborn/preeminent one over all creation, that all things (all things!!!) were created by and for him, that he is before all things, and that all the fullness of God dwelled in him. In Colossians 2:9 Paul tells us that in Christ all the fullness of deity dwelled in bodily form. We also have Titus 2:13 in which Paul describes Jesus as “our great God and Savior.” Many more could be cited. That Paul considered Jesus to be deity is well established, with Philippians 2:6-9 being only one of many.
As for Philippians 2:6, I agree with your analysis of the Greek word morphe, which means shape or form. Paul could have used other words, but the passage makes it very clear what Paul is talking about. Later in the passage he tells us that at his name every knee will bow and tongue confess. The Bible only allows bowing to God himself, so Paul is clearly implying deity in Christ here. Paul also tells us that Jesus was made in human likeness, which means that beforehand he was not. Besides, God is not a physical being and he does not have a “shape.” Therefore, the world morphe must be used metaphorically. I looked up the Greek word and it is variously translated as form, outward appearance, nature or character. Given the context, in which Paul tells us that Jesus took on the morphe of God, outer appearance would not be a good translation. Probably form is a better translation than nature here, but the point is clear. Jesus had the nature of or form of God. Then there is the word harpagmos which means held onto or grasped. I looked into my Greek dictionary and I definitely did not see your translation, which is siezed or stolen. The root word harpage has this meaning, but not the word harpagmos. There are two possible implications of this word. It can mean grasped, as in the sense of acquired/attained or it can mean grasped as in not let go of/clung onto. So, let us look at the context of Philippians 2:6-9. Is Paul talking about something that Jesus tried to hold onto or is he talking about something Jesus is letting go of. Look at Philippians 2:7a where Paul tells us that “he made himself nothing.” From the context, it is quite clear that he is talking about something that Jesus decided not not to hang on to or something that he let go of. It does NOT mean that Jesus was trying to grasp equality with God, but that he was letting go. Therefore, I believe that the standard interpretation of Philippians 2:6-9, which is that Jesus was by his very nature or character God and that he did not cling onto that nature is the natural and best translation of this passage. Could it have this opposite meaning? Yes it could, but the immediate context leans another way, in my opinion.
Then we can look at Paul’s letters more broadly and even the entire New Testament to decide if Paul is talking about Jesus clinging onto his God-likeness or if he is trying to attain to God-likeness. Again, I will claim that the tenor of Paul’s writings and conclude that in his incarnation Jesus let go of his deity to become human. I agree with you that both the words morphe and harpagmos can have a range of meaning, but I believe that the common translations get the sense of it correctly. You say that the varied translations are “working hard to try to make a word that depicts Jesus as not being equal with God.” I have to disagree with you on that. We have a boat load of passages from Paul which prove he saw Jesus as divine (a few quoted above), and none that show Paul thinking of Jesus as merely human, so I cannot agree with your conclusion. Jesus said of himself, “Before Abraham was born, I AM.” Jesus said “I am the resurrection and the life.” Jesus said that if you have seen me you have seen the Father. All of this lends credence to interpreting Phil 2:7 to imply that Jesus was willing to let go of his God-nature to take on a human nature.
I know that some disagree, but that is how I see it.