TBH don’t worry too much about good boots. It’s good to just have a pair of boots for when you actually plan to be out in the snow, but unless you do a lot of hiking (where you’d probably want a pair of boots even in the summer) you’ll likely find that you won’t truly spend that much time trudging through snow. And it sounds like you don’t even anticipate getting all that much of it either.

I just have a few pairs of insulated and waterproof socks that I wear when going out in winter and just wear my usual sneakers with them. Sneakers that actually have traction though, none of those flat-soled Vans or similar, or you’ll slip on ice. Ice will be a bigger concern than snow.

Easier to build a bridge over a freeway that already exists than to divert the freeway into a tunnel, I guess. Tunnels also require solid bedrock for structural integrity, which for all I know, this region may not have. And there may be additional risks in California with the frequency of earthquakes. But I’m not an expert, just assuming that they already weighed options and had reasons to settle on this approach.