Question 1- depends on any other language. There are 5 factors in land ownership, and if they kept any executory rights, they would have a royalty interest that could participate in new leases and bonuses. Absent that, they would have an NPRI, and would receive the portion stated if oil and gas are produced.
Question 2- again, what you have provided is a percent ownership in some number of acres. The number .042 X 3/16 would describe the net revenue interest where 3/16 is the royalty rate, and the .042 is the owner interest.
2 (b), not an attorney, but they are buying a royalty payment stream. More correctly, you do have a .042 interest, which you show as 3.36 nma, and at a 3/16 royalty rate, provides a .0079 net revenue interest. if you had that land under a lease with a 25% royalty rate, you would still have a 0.042 interest, but your net revenue interest would then be a 0.0105 net revenue interest. The net revenue interest is what the tax assessors base their appraisal.
Question 3--I think the 6.72 you showed is a mistake. In the S/2 of 80 acres you would have an apparent 3.36 nma in the lease. Make sure that all of your property is physically within the oil and gas lease, as that changes the .042 and corresponding Net Mineral Acres, since you are using the lease (apparently).
3(b) -- the .96 is actually the factor to determine what you actually own in that section. Make sure that the highway ROW includes minerals, as i have several properties that the ROW is a surface only easement, and I still own the minerals beneath it. Again there are several terms that can describe you interest--net mineral acres or royalty acres are synonymous, but prefer NMA.
Question 4--there are a number of good oil and gas attorneys in Austin, one of which is John McFarland at Graves Dougherty. I don't believe you gain anything with a Midland attorney, frankly.