significance in McGonagall’s years of service?

[elizabeth1603]

No: HPFGUIDX 78114

This turned out to be a longish post.

I've checked the archives, and I can't find anything on this subject. Please 
excuse me if I'm wrong on that. 

Umbridge had many uses in OOP, and I think one was to provide information 
about other characters. One way she did that was by interviewing the 
teachers while she was high inquisitor. 

We find out that Snape began teaching fourteen years ago and that he keeps 
applying for the DADA position. None of that is very enlightening. We know he 
wants that job, and we can assume that he took the potions position upon the 
disappearance of Voldemort, after he proved himself loyal to Dumbledore. 

We also find out that Professor Trelawney has been at Hogwarts for sixteen 
years, but later on, we find out the story behind her hire as well.

Now, McGonagall's answer is what I find intriguing. We learn that she has 
been at Hogwarts for thirty-nine years, but we don't find out any new 
information behind her arrival at Hogwarts. Is this information that we're 
supposed to keep in the back of our minds for a later purpose?

I have my own opinions about McGonagall. She came to Hogwarts twenty-
four years before Harry was born. I believe she is Harry's great-grandmother 
on James's side, and that, for some reason (perhaps James's father was 
killed), it became necessary for her to take on a job to help the family out 
when James was born. 

Now, I know this theory doesn't fall in line with the "James was well-off" 
theory, and it's a little far-fetched besides. What do you think?

Elli