How do Seventh Day Adventists understand "a form of [a] God" in Philippians 2:6? SDA is Trinitarian and believe that Jesus in his preexistence was Michael the archangel. Do they then believe that Jesus' preexistent form was that of an angel all the time or that Michael was a form he adopted when doing messenger work and such? In other words is Jesus an angel reincarnated as a man or some other "divine form" incarnated as a man?

And what do they say going on with Michael waging war in heaven post-resurrection?
Hi let me start by correcting some of the assumptions you seem to have made


  Jesus in his preexistence was Michael the archangel


This statement suggests that there was a change from Michael to Jesus. In fact Adventists would say that Jesus is Michael and that it is another name for Jesus. When you look at the name Michael, it has the meaning of "One who is like God" so it fits the character of Christ. 

In his pre-existence, Christ was involved in many aspects of earth's history including creation. Note the plural pronouns used when God says Let us make man in our image.  

Next is the assumption that Michael is an angel. The term arch angel is a transliteration of ἀρχάγγελος which is translated chief of the angels. So it is more that he has authority over angels rather then being one. 

This should resolve the first two questions. Michael/Jesus is the Son of God incarnated as man. 


  what do they say going on with Michael waging war in heaven post-resurrection?


The assumption made here is that this is post resurrection. Adventists would describe this as the beginning of the great controversy and that this actually happened before the fall. This is a brief summary of the interpretation of Revelation 12 and you will see that this is not interpreted chronologically as it appears.


The war in heaven. This is the initial disagreement of Satan against God. He is cast out of heaven to earth taking some angels with him. At this point he, Satan, is on earth and can tempt Eve. 
The woman who gives birth is God's chosen people giving birth to Christ, who was crucified and caught up to God again. 
The history of the church is then explained using symbolic language. 


A lot of these answers don't have the substantial amount of work that is required to complete this story.  I can provide more detail around sections if needed.