1/ Exciting stuff! We built the hyperliquid chain using tendermint consensus but otherwise written in rust from the ground up. No hate on go/cosmos, we just needed maximum TPS and lowest block latency. We also recently got IBC working between hyperliquid and a cosmos chain.
2/ Code's not quite ready for open source yet, but let me know if anyone's interested or has questions. We weren't sure it was feasible without the Cosmos SDK (lots of code there) but indeed it does work. Like the OP said, IBC really is chain agnostic at a practical level.
3/ We're super happy with the tradeoff of building directly on tendermint. It's more work for us but the ability to optimize more of the stack is amazing. IBC being pretty painless to set up is the icing on the top.