There are a lot of ways to skin a cat. After being on this forum for years I have learned how to turn the other cheek. I may have upset you a few years ago about headstock alignment and I apologize. I missed your information. I sometimes can be a bit of a pain I know, but I hate seeing info that as a professional rebuilder would not do. That's my opinion. Your way worked for you and you were trying to help others. We all need to learn from our mistakes and not get mad at each other, especially when all can share our lifetime of experiences. I would love to tour your shop and spend days learning about your wood working and machine shop. Stephen please continue to contribute in the Rebuilding forum. The forum has lost so many because people could not turn the other cheek. Charles has done an extremely fine job patrolling the forum and getting rid of the trolls.This is why i stopped posting about rebuilding subjects, and will take this as a hint to stop again, going forward.
The gages on the inverted Vee's were the point of illustration, to show where/how you keep track of deviation in an inverted way. It is applicable to the OP's inspection as contrast to leveling over the flats on top of inverted Vee's which are not a reference surface. They don't wear at the same rate or in the same way, as the rest of the inverted Vee, and provide no useful information.
The photo's i had available show the use with the lathe bed on the planer, and give some ideas to how the references can be used.
To answer your first Q, the planer tool is being adjusted, before using to plane the underside flat of the back way. But it does not matter what the purpose is. Concentrate on a method, any method, to keep track of deviation in the inverted Vee's, and then reference that to other cross sectional features in the bed.
Hang in here please as I have had to endure some rude people and still love to help pass on the lost art of my trade. I had to put a number of the trolls on ignore. It keeps my blood pressure down. Thanks again Smt. Richard King