1. The purpose is to get good ground continuity all around the PCB
2. It was 100% routed by hand since when I started the project Kicad wasn’t mature enough (at least in my opinion) so that I could have comfortably relied on autorouting result. Of course it might have been enough to hand-route only the critical signals, but on a PCB of that size it wasn’t huge amount of work to route everything manually.
3. I’ll be trying ADV7513 with cps2_digiav project, but ideally HDMI TX would be implemented on a higher-end FPGA that has suitable transceivers. Unfortunately such solution requires HDMI subsystem IP that’s not freely available as far as I know. I’m not sure if any new dedicated HDMI chips (or their documentation) will be publicly available in the future, making things much harder for hobbyist projects like this. Even ADV7513 is no more openly available on Mouser.
4. I’m not big fan of 3.5mm connectors either, but the purpose was to keep PCB size minimal.