To disconnect those leads you need first to unplug the A board from the neck of the tube, and then to disconnect the plastic plug of those leads from the main neck plug of the A board (2 clips on the sides). But for some quick soldering job on the D board it's not really necessary to remove that plug or even the flyback plug, just let the board sit on its heatsinks, standing on a high cardboard box next to the screen. Now, about the root of your issue, it's hard to tell. As far as I know an image squished in the center of the screen should mean horizontal S-correction capacitors with capacitance too low or drifting in another way (that's something I have on my "to check" list as my "guinea pig" GDM-5410 has that issue). But yours is the contrary, and film capacitors never increase in capacitance with use. Moreover it should be a stable issue, not something eratic. I think you should check the voltages output on the G board first before messing with anything else. That can be done easily with a common multimeter, putting the red probe on the exposed metallic part of the top plugs, and the black one on the chassis (warning, make sure to only hit one single pin at a time, or you'll make a short and blow fuse resistors). I've discovered there might be a design fault on the 15V rail, the rectifier bridge seems undersized or sized too close to the current needs. With aging it may not be able to provide enough current anymore, and heat more and more, which is the infernal circle leading to the death of such component.