Box 1: Yes -
Use the set-backend-service policy to redirect an incoming request to a different backend than the one specified in the API settings for that operation. Syntax:
<set-backend-service base-url="base URL of the backend service" />
Box 2: No -
The condition is on 512k, not on 256k.
Box 3: No -
The set-backend-service policy changes the backend service base URL of the incoming request to the one specified in the policy.
Reference:
https://docs.microsoft.com/en-us/azure/api-management/api-management-transformation-policies
