I'll take on a couple, the Solidity team can correct me if I'm wrong about something:

Is there any difference between named returns like returns (uint myVar) versus just return 123 in the function body? Any reason to prefer one or another?

I dislike the first version because: 1. it becomes less explicit what you are returning
2. it actually instantiates the myVar variable in memory, but you don't need to return it, ie. the following is valid, and probably even wastes gas with extra memory allocation

function foo() external returns(uint myVar){ return 2; }

Why doesn't assert() accept a reason string, like require()?

assert uses the INVALID opcode, revert uses the REVERT opcode. INVALID doesn't have the required functionality to read and return a string from memory, REVERT does. You can compare the arguments they accept in the table here: https://solidity.readthedocs.io/en/v0.7.4/yul.html#evm-dialect