GCP Load balancer URL map: Distribute traffic by header and everything non-matching via location

Question

I am trying to set-up URL maps for GCP load balancers. I have Service C for static files and the two web application services A and B. C should get everything not starting with a certain path, the rest if the traffic should be divided by A and B depending on headers. If there are no headers, then the location should be decisive. A and B live in different regions but also have some different logic.

In sudo code, the URL map would be something like:

If request does not contain path /my-path then go to service C

If request contains path /my-path:
- If requests contains header A1 then go to Service A
- If requests contains header A2 then go to Service A

- If requests contains header B1 then go to Service B
- If requests contains header B2 then go to Service B

- If request contains none of headers A1, A2, B1, or B2 
then go to either Service A or Service B, 
preferable to the service locally closer to the origin of the request.

What I've got so far is:

defaultService: projects/my-project/global/backendServices/service-c
name: path-matcher-1
routeRules:

- description: A1 to A
  matchRules:
  - headerMatches:
    - headerName: A1
      prefixMatch: ...
    prefixMatch: /my-path
  priority: 1000
  service: projects/my-project/global/backendServices/service-a

- description: A2 to A
  matchRules:
  - headerMatches:
    - exactMatch: ...
      headerName: A2
    prefixMatch: /my-path
  priority: 2000
  service: projects/my-project/global/backendServices/service-a

- description: B1 to B
  matchRules:
  - headerMatches:
    - headerName: B1
      prefixMatch: ...
    prefixMatch: /my-path
  priority: 3000
  service: projects/my-project/global/backendServices/service-b

- description: B2 to B
  matchRules:
  - headerMatches:
    - exactMatch: ...
      headerName: B2
    prefixMatch: /my-path
  priority: 4000
  service: projects/my-project/global/backendServices/service-b

- description: Distribute everything else to closest location
  matchRules:
  - prefixMatch: /my-path
  priority: 5000
  service: projects/my-project/global/backendServices/service-a
  ....
  # TODO!

(I have omitted the exact header name and values. Those work in general and do not matter in this context I think.)

Now I have two questions:

1. How do I implement the last part like distribute preferable by location, i.e. service whose region is closest to the user's IP?

2. Does the rest of my code achieve what I want it to do? Seems so according to my tests but not 100% sure here