No, I never heard
of the diagonal line thing either- I invented it, after looking at bikes that seemed to have their names in the 'right' place and trying to work out why.. Try it on your pic. of your bike at top of thread in it's Poyner finish, or on Hillwalker's Quinn, for example. I'm not saying it's the rule, it's just the best I can come up with. You may come up with a better one. I guess there must be aesthetic rules for the placement of seat-tube and other decals as well, either learnt, or intuited, by people in the trade who do this professionally. Nothing wrong with breaking the rules, either, IMO, but if you do, it helps to know
you are breaking them and have a justification...
Anyway, the fact that you confessed your misgivings to your mate reveals that you are sensitive to this placement as well...
) advice to anyone who is repainting a frame with thin-walled tubes is to either do it yourself, or take it to someone (you know the names) who, you can be sure, understands bike-tubing. Gaspipe and such can stand up for itself at the blasters/powder coaters, ageing DB might need a lot more care, especially if you don't know how many times it may have undergone the refinishing process already.
Regarding the rear wheel, I think the rim needs to move over to the non-drive-side, yes? Assuming your frame hasn't been 'sprung' to take a hub with longer OLN, if your chainline is good, and your smallest cog is pretty much next to the dropout, (with just enough room to shift the chain onto and off of it) that is ideal.
In that case I would slacken off all the drive-side spokes a quarter-turn, and then tighten all the non-drive-side a quarter-turn. Keep doing that until the rim is centred. But you need to stop if you run out of spoke-thread on the non-drive-side (nipples will turn no further), or start exposing spoke-thread beyond the nipples on the drive-side. If it can be done, it should end up a stronger wheel with a bit less dish, as a bonus.
That's the easy way. to be really
thorough you have to get the block off and the tub off, I guess... sooner you than me..