Need help with Power Query Add Column using multiple conditions

Hello, and thank you for reading:

There are 2 conditions that need to be met, for an end result.

if [priority] = "1 - Critical" and [major_incident] = "TRUE"
then "P1"
else if [priority] = "1 - Critical" and [major_incident] = "FALSE"
then "P5"
else null

My second statement "else if" overrides the first statement. What am I doing wrong?

More detail:

I ran the first statement alone, and it works as expected. It gives me the end result of "P1". However, when I run both the statements it replaces every row with "null" - and ignores the fact that the conditions for the first statement WERE met, but didn't reflect in my data.

How do I change my query to accomodate the first statement? 

Even more detail if needed:

This question will be called a success if I can accomplish the end goal of:
If a P1 is accompanied by a TRUE Major incident = P1
If a P1 is accompanied by a FALSE Major incident = P5

If a P2 is accompanied by a TRUE Major incident = P2

If a P2 is accompanied by a FALSE Major incident = P5. 

So on, so forth... all the way to a P4. And it has to be done all within 1 custom conditional column with 1 query. 
ANY time "major incident" is False, the end result must be "P5" 

Thank you for reading!! Any help here would be appreciated. I feel like I am very close but I have ran out of ideas.