Always good to check the voltages rails first which I see you did. But I would still be interested in figuring out why that 5V rail is so out of spec (should be 5 +-0.075V). Also, I would check the ripple on those power rails as well. It should be very low (~2mV Vpp).
I really don't understand why people feel the need to replace all caps just because the unit is old. I'm glad it didn't cause any further issues because that can be one of the downsides to doing that.
Why do you suspect the issue being in the preamp section? Does the anomally appear in signals for both CH1 and CH2? Both channels have their own preamp circuitry so if the anomally is appearing on both channels, I would suspect the issue being further downstream of the inputs. Possibly the vertical output amplifer.
I would also be looking at the CRT circuitry. In some of the photos you show, the trace rotation is clearly off. It's possible you might have some issues with geometry of the display.
You have the right idea by injecting signal and following it thru the vertical amp circuitry to the CRT. If the issue is there, with the help of the service manual, you'll be likely to find it.
I've never owned or worked on a 465B, but I'm sure there are plenty of people in this group who have that will be able to give you more specific suggestions. Also, might be helpful to search through the forum to see if this issue has popped up before. Use the search box on the top right.
Good luck!
-Frank
Frank Mashockie