You missed the point entirely.
20 hams, some whom had never built anything will soon have 100% scratch-built SSB transceivers on the air. The design goal was a low part count, common component, easy to assemble rig that performs well on the air. There is no "complete schematic" by intent. This is a project for for inexperienced builders and we take it one module at a time. I don't need the full rig on one piece of paper in order to build the audio amplifier stage. In fact, if I had had to start with a full schematic I would never have picked up a soldering iron. But if you really want a full schematic, in the spirit of a community project go ahead and make one.
Similarly the decision to use a commercial filter and mixers was a design decision to increase the likelihood of success for the first time builder. You could say the same about the SI-5351 and Arduino. The decision to use relays for steering is also a design choice. Reliability has not been an issue to-date and if it is, they are easy to swap out.
As far as citing sources - most of the modules are pretty standard circuits that derive from many sources. The bi-directional IF module is decidedly not a ubitx derivative. The bidirectional amplifiers that comprise it are documented in EMRFD and were originally in a Plessey Manpack.
The result is a fine performing transceiver - I have worked much of the world SSB phone on 5 watts - and the whole world on WSPR. I get great audio reports and the rig produces a very clean and compliant spectrum.
It is also an experimenters platform. The modular design makes it easy to try different design ideas - you don't like the relay steered IF - substitute your favorite circuit and share it. Since my original build I have added CAT control via software, made it a two bander (I used relays for the BPF and LPF switches also), Added and S-meter and audio derived gain control. And the sketch now supports LCD's, color TFT's or Nextion displays.