Here’s a non-expert opinion and probably not what you are hoping to hear.
In Texas lack of leasing or drilling activity for any period of time doesn’t extinguish mineral rights. In other words, if the chain of title to your property includes reservations of mineral interests they remain reserved and won’t revert to you as the surface owner. The law is different in Louisiana but that’s the case in Texas. The only way you could gain control of those previously reserved mineral rights would be to buy them from whoever currently owns them.
Determining what portion of the mineral interest, if any, passed to you when you acquired the property requires reviewing all of the documents in your chain of title and totaling all of those partial interests that were previously reserved. If your tract was originally part of a larger piece of land and different reservations applied to different parts of that original tract it makes the job of determining what applies to your tract harder but that’s what it takes. It’s also important to determine whether the interest that was reserved was a mineral interest or just a royalty interest. Like you said, reservations made years ago by now may have passed through several generations of heirs who may not be aware they own them and be difficult, if not impossible, to locate. They still own the reserved interest. In cases where there is production and heirs can not be located the State of Texas ends up receiving and holding their share of the royalties.
It sounds like the research you have already done may give you a pretty good handle on how much interest has previously been reserved. If not, that’s the sort of thing petroleum landmen work on everyday. Depending on the size and value of your property, having a landman, or attorney, review your chain of title may be what will be required to pin down what is involved.
On your question about whether someone you sell to should be concerned about the status of surface control to your property, in my opinion they should be. Unless those prior reservations were royalty reservations which don’t impact surface control, or the previous mineral reservations included wording waiving surface use related to those reservations, which would have been unusual years ago, then you don’t have full surface control and therefore couldn’t pass full surface control to a buyer. That title research mentioned earlier would be able to determine your degree of surface control. Depending on the type property you are talking about many potential buyers don’t know enough about minerals to question the status. A knowledgeable buyer may be willing to accept it with little or no mineral interest but if you don’t own at least 50% surface control probably won’t be willing to close unless a waiver of surface use could be obtained from whoever owns the majority mineral interest, or would require a concession on the sales price in return for accepting that risk.