Q:

Motion verbs and ところ

As I understand it, something like 帰るところだ usually means you are in the process of going home, say on the train. But ところ often has the meaning of "just about to do something."
Does something like 帰るところだ also have this meaning? e.g. Can you say it if you are still in the office and about to leave in a couple minutes?
If so, my main question is I'm wondering if ところ always has this ambiguity (at least from the English perspective) between meaning both "in the process of doing" and "just about to do"? Or is it something special to motion verbs and other verbs that are sometimes called stative verbs?
What I mean by stative is 帰っている does not mean "is in the process of going" it means "went home and is now there." I'm wondering if ところ just seems to have two meanings in English because when you sitting on the train going home you are still "just about to go home" from the perspective of Japanese because 帰る is stative and you haven't completely arrived yet.
Basically I'm wondering if this is correct:　
雨が降るところだ = "It is just about to rain." (NEVER "It is in the process of raining.")
雨が降っているところだ = "It is in the process of raining."
帰るところだ = "I'm just about to go home (at the office)." OR "I'm in the process of going home (on the train)."
And I would guess 帰っているところだ sounds strange and doesn't make much sense(?)

A:

When it's するところ, it means "about to do (something)."
したところ means someone has just done something.
しているところ means someone is doing something.
していたところ means someone has been doing something. 
Literally, ところ means 'place', but it's also used for a figurative place as this dictionary page defines it as 

2　抽象的な場所。  

オ　ちょうどその所「さっき着いたところだ」  

Also this page puts it as

形式名詞 
「こと・の・ところ・ほう・わけ・はず・つもり」など、  

今テレビを見ているところです。　

Now

Does something like 帰るところだ also have this meaning? e.g. Can you say it if you are still in the office and about to leave in a couple minutes?  

Yes, you can say that when you are still at the office but about to go home.  

And is the reason that 帰るところだ can be interpreted as "in the process of going home" because motion verbs don't complete until you reach the destination?  

Theoretically, yes. But if we are just in front of our home, then we probably choose a different expression such as 今もう家の前にいるんだ or もう今家に着くところなんだ.  

[replying to additional request from the OP]  

雨が降るところだ = "It is just about to rain." (NEVER "It is in the process of raining.")  

雨が降るところだ is an unusual thing to say. To mean "It is just about to rain," we likely say 雨が降りそうだ.  

雨が降っているところだ = "It is in the process of raining."  

This is not bad, however we more likely say 雨が降っている.  

帰るところだ = "I'm just about to go home (at the office)." OR "I'm in the process of going home (on the train)."  

Yes, these are correct.  

And I would guess 帰っているところだ sounds strange and doesn't make much sense(?)  

Doesn't sound so bad if you are on the train, but if you are still at the office, it does sound strange.  

What I mean by stative is 帰っている does not mean "is in the process of going" it means "went home and is now there."  

Right. I am aware that Japanese continuation form ている is actually more of state than progression.  I find that is why we need ところ to make it progressive.

I'm wondering if ところ just seems to have two meanings in English
  because when you sitting on the train going home you are still "just
  about to go home" from the perspective of Japanese because 帰る is
  stative and you haven't completely arrived yet.  

I think 帰る itself is not necessarily stative. The form ている makes it stative.

