You are not alone in being confused about this. I know that there are individuals doing genealogy who are comfortable with the word "witness" with respect to sharing facts. I am not one of them. I would just like to banish it from use. For example, the Midwife who attended a birth was the midwife for the birth, not a witness. The Pallbearers who served that role at a funeral were pallbearers for the funeral, not witnesses. The Flower Girl at a a wedding was a midwife at the wedding, not a witness.
I do think there are legitimate uses for the word witness. Wills have witnesses when wills are signed. There are usually two official witnesses who sign some sort of paperwork for a marriage. Deeds often are witnessed. Etc. But to me this is a specific legal function, required by law. So I like the term Role much better than the word Witness for shared facts. The word Witness for shared facts just grates on me like the sound of fingernails on a blackboard.
That being said, probate usually does include witnesses in the legal sense of the word, in which case the Role is Witness, just like a Role can be Midwife or Flower GIrl or Pallbearer. Without seeing the text of your document, I can't tell what Role we ought to assign to Gladys. It seems likely it would be Primary_Heir, but it could be something else.
A good way to think about the problem is as follows. Suppose you weren't using RM at all, nor any other genealogy software. Suppose you were just using a word processor and were typing in a little family history in your own words. What would you say? It might say something like the following.
John Doe's will was probated on 12 Jul 1932. His wife Gladys was the primary heir to the estate. His children William Doe and Sarah Doe Johnson received $1,000 each, with the farm and all the rest of the estate going to Gladys.
Well, that's a really wild guess about how you would want it to read. But however you would want it to read with a word processor, make it read the same way with RM. In this case, you might have roles such as Primary_Heir and Other_Heir. Then your sentence could be something like the following.
[Person:Possessive] will was probated on [Date:Plain]. [Primary_Heir] was the primary heir to the estate. His children [Other_Heir] received $1,000 each, with the farm and all the rest going to [Primary_Heir].
To get this level of customization, you would have to customize the sentence just for John Doe. With more thought, it might be possible to come up with a more generic sentence that wouldn't require customization for each probate situation in your database. In any case, the Probate fact would go to John Doe. The Primary_Heir role would be assigned to Gladys. The Other_Heir role would be assigned to William and Sarah.
This is not a complete solution for your situation, but it should get you started in the right direction.