1)
There are many ways to approach this. The simplest to start your researching on would be "valence shell expansion". That is a super simplified version, but you can get info and go from there, and ask if you want/need to go further/deeper.
2)
Well, I am confused by part of your question.
Why does phosphoryl chloride have a double bond with oxygen?
Would the Oxygen be happy with just a single bond? Would it be charged, and if so, how? Is that stable?
Couldn't it just make 4 single bonds with the 1 oxygen and 3 chlorines?
Say what? 4 single bonds with 1 oxygen? How are those then single bonds?
That whole part of your question is rather confusing.