1. The FE-405B firmware looks like it's working reasonably well. The oscillator isn't really stable enough for the extra-long time constants that the 5680A can have. The current firmware has 100 seconds, 15 minutes and 1/2 hour as the three modes. I'm going to add the source to github shortly. I had to stop testing it, though, since there's enough RF leakage at the right frequency to jam our garage door opener remotes and home alarm system.

2. The divide-by-3 circuit had a small error on the board, but having worked around that, it worked. I was able to get a 5 MHz square wave out given 15 MHz in. I'm ordering new boards, but mostly out of curiosity.

3. Adding a simple 3 element pi network low-pass filter to the output results in a nice, 13 dBm sine wave. The component values I used were 2 220 pF caps with a 1.2 µH inductor between them. I'm going to add footprints for these components on one of the outputs of all the variants, so you can choose between square and sine outputs as you wish (note that the two are not likely to be in phase). If you instead prefer two square wave outputs, you can replace the inductor with a 0Ω resistor and remove the two caps.