Heheh! Wish I could tell you how much stock I’ve gone through in the learning process. Definitely in excess of $1000 worth by now. (You could probably ask Glowforge, they likely have records of it from the PRU days.) 
Well, for now, I don’t think you have a problem with the lens or the printer head. If the laser isn’t cutting with enough power, the first thing to do is to check that everything has been cleaned- you now know what to look for, but it helps to just do a little maintenance clean once in a while to keep it from building up in the first place.
The other thing to check is to make sure that the material is completely flat. (I like to use the Honeycomb Pins - I’ve never had good enough results with magnets.)
It won’t hurt to keep using the 155 speed setting for your walnut ply. If the other Proofgrade settings aren’t cutting through either, you can lower the speed on those by five or ten points. I generally set my own speed and power settings for materials.
And even with that I still sometimes have something fail to cut through with the default settings. Doesn’t happen all the time, just once in a while. But I always pin my materials down, check them with a pick like one in this set to see if they cut through before removing the print. Then I can send a second cut if I need to, and I don’t lose as much material these days.
Support is going to need to look into anything else it might be from their end. So they might have some other tests for you to run, but hopefully this will let you get your client jobs finished in time.