I would think there is more than $4450 in suspense, but it’s hard to know for sure without knowing exactly what you own now. There are a handful of companies that chase after “missing owners”, and they’re willing to do a lot of the leg work (they have to in order to get put into pay by the oil company). They have probably already made a guess at what you own, and will want to buy all of it, including the suspended royalty payments. But, they’ll also likely need your help (or someone in the family) to “prove” the ownership from your grandfather to you.
You are correct that an Affidavit of Heirship may be necessary. Luckily, you should not have to step foot in Howard County. The Affidavit of Heirship is typically used when the family member(s) died intestate, without a Will. If there was a Will that was admitted for probate, then your process will be different. You will need all that documentation. But, if that is not the case, the Affidavit should show the family history of your grandparents, then on to your parents, and how you are entitled to X percentage. Ideally, somebody with knowledge of the family history who doesn’t stand to gain from this would be the person signing the Affidavit.
If you want to sell, I’d suggest you get it all sorted out with Sabalo first. Work with them to figure out what is actually owned, get paid up for any past royalties, and then figure out what a reasonable price would be for the minerals. Or, just have it put on direct deposit (if they offer that), and collect the income as it comes in. If you don’t need the money now, it would likely be best to wait and sell in the future when commodity prices are higher.