I edited your quote to answer the best I can on three points.
On the first point, I'm quite sure. Many moons ago I cleaned and de-burred beds ways, including the sides and bottom. Outer flat that I can mic directly was within .0004" the length of unwore bed from the underside. Its so close, that I would presume both front and rear bottom sides are done the same way. To the micron, surely no, but even if I'm extremely generous and allow a +/- .0005" variation on each reading, it won't significantly change the assessment. And my readings are more accurate than +/- .0005".
On measuring the flat way, yes I'm using the bottom as reference. But on vee way, I'll call it maybe 50%, as I'm coming in nearly on the corner. So the other half, is the side of ways, where I measured +/- .001" checking various spots the entire length on bed. Better seen from this angle:
View attachment 340446
I had also wanted to measure the bottom side to crown of vee, not that I entirely trust that measure, but curious to see the numbers. But the underside does not go in far enough. Its close, and looking at the end, it looks like you might be able to mic it, but you can't, mic won't sit straight.
View attachment 340447
Another reason I trust my numbers, is my eyeballs and fingers work a little bit too

. Not as accurate as a mic, but I can tell the difference between .001" and .010". Before putting an indicator or mic on it, I guessed 10 to 15 thou at the heavy wear spot.
Here's a look at the low/no wear section near TS:
View attachment 340448
Now here's a look at the area where I'm down about .010":
View attachment 340449
On your 2nd point of the Kingway tool. I'm going to quote Harry Bloom. You can find that in this post:
Another New Toy
I'm going to say my opinion is a little more inline with his. Not that I'm dumping on anyone, or the tool, its just not part of the system I'm doing. And without trying to make a big thing of it, I think some get so far down that rabbit hole of rebuilding, that it creates some confusion and paralysis. Where it might get hard to see the forest with all the damn trees in the way

.
Yes, I can run the machine with ways as they are. In fact I'm pretty fair at using worn out crap. But on this I'm looking to increase my contact areas for greater rigidity, and improve the accuracy.
In dealing with the wear, I'm treating both the flat and the vee surface the same as I would set up a surface grinder. If I can contact both ends of the work piece(the bed), to the same height during a tramming operation, then I will gradually work the un-wore sections down evenly until I reach the bottom of the valley:
View attachment 340450
The only reason I will abort now is if I can not tram up and down the ways for a consistent reading.
P.S. If you guys could do me a favor, don't quote my posts in their entirety. It eats up pages, and makes terrible readings. Quote the portion you want to comment on. Appreciate it.