No option to decouple

SAS is your problem. If you turn it off, the oscillations should die out eventually. Additionally, you should be able to stop the oscillations using the time acceleration trick. The reason that the oscillations are increasing to destructive levels is that the parameters that control the SAS module are tuned to the natural resonance frequency of the station such that corrections are causing a positive feedback on the motion. Unfortunately, there isn't a way in the stock game to tune the parameters for optimal performance (what is known in control system parlance as critically-damped).

There is hope though. If you're using the Kerbal Attachment System (KAS) and Kerbal Inventory System (KIS) mods (they're best installed as a pair, but KAS is the only one you really need), you can attach struts to stiffen the solar panel beam relative to the rest of the station. Additionally, you can also change the resonant frequency of your station by adding to it. In particular, structures extending from the problem axis (in this case, the roll axis) will have the most effect.

Also, I can't tell if this is the case or not, but you only want a single SAS module active at any time on a non-rigid structure, or at the very least, have all the SAS modules on a single subsection of the station. At the very least, this will reduce the torque that's applied to the station, reducing the possibility of having an undamped system.

Well, right on top it says "Porkchop selection", and that is what it shows, a porkchop plot.

It plots launch date vs. flight time on the axes, and the color represents the needed delta-v. Blue is lower delta-v, and red is higher delta-v.

This means that the more right you go on the plot, the later is your departure, and the higher you go the longer is the flight. Selecting blue areas means you need less delta-v.

The "Lowest dv" button automatically chooses the most delta-v efficient transfer.

You can also scroll into he plot to see some areas in greater detail using the mouse wheel.