If the GPS has the required satellite coverage for RAIM, it will scale, if it doesn't it won't scale, simple as that. It doesn't matter what the almanac says.
How does the receiver predict RAIM will be available for 5 minutes after passing the FAF without knowing where the satellites will be in the constellation?
I thought it would use the almanac?
I've seen units fail to go to approach mode/approach active even though RAIM was available. Poor satellite geometry relative to the ships GPS aerial might cause this.
How do you know RAIM was available at the time?
Why would you bother if you had the RAIM prediction from ASA or Airways. The RAIM prediction from the ships GPS unit isn't going to tell you anything that you don't know. As has been already said the ASA or Airways prediction is going to be more accurate anyway.
I think you've answered your own question.