I've managed to vastly improve the power output of the SHG by doing a little cleaning inside the box.
After some consideration I decided that the only remaining possibility was something wrong inside the SHG. Its a hermetically sealed box so I cleaned around the area (to keep dust down) and then removed the lid. Some of the screws were loose, so its likely that somebody has opened this before. Below are a few pictures of the inside of the Diabolo SHG.
Looking through an IR viewer I could see that there was a large absorber on the front face of the crystal. I didn't get a picture but it was dead center and scattering a lot of light given the tightness of the beam. There was also other smaller scatters around the edge of the front surface. An clean N2 ionizing air gun was used to blow in and around the cavity and its electronics. There was a lot of dust blown out from inside the unit; the SHG has evidently been opened at some point in a dirty lab. The spot in the middle of the front face of the SHG came loose with very gentle bushing with a corner of a lens cleaning tissue (MC-5). I thoroughly flushed the inside of the unit again with 40 psi clean N2 and then replaced the lid. Just before sealing the box I cracked the lid and flushed again with the N2 gun to fill the box with clean dry air. The screws were sealed down with 2.25 Nm of torque.
I ramped the laser back on and after some small small alignment tweaks I was able to get an initial large boost in 532 nm laser output but it quickly dropped to the lower ~12 mW output again. It looks like, after lock, the output light goes through a few fringes and then either settles on a HOM or a weaker fundamental mode. My guess is that in the locked state the crystal/mirror is heating up and mudslinging the cavity. I don't really want to pull the unit apart into is constituent pieces to inspect. I might need to have a closer look at the crystal front face absorber and the state of the input coupler mirror. Over time some non-linear crystals do develop either grey tracking or permanent damage along the tighly focused points of 532 nm light. It could be that the unit has gone past a tipping point but this is hard to see from just peaking in from the top of the unit.
Not sure what to do next.
Side note: Its seems very hard to make any alignment changes from the inside: my guess is that alignment is done from the three outside exposed hex screws on the front coupler. Its unlikely with such a short cavity much alignment changes would be needed. For now its best to leave it as it is.