I'm glad you're interested! The world could use more people tackling spam.
I'm not going to discuss signals for obvious reasons. Suffice it to say web browsers are very complex pieces of software and attackers are often constrained in ways you might not expect. There are many interesting things you can do.
I have no idea how much effort Google will make to support old browsers going forward, sorry. To be double-super-clear, I haven't worked there for quite a while now. Over time the world is moving to what big enterprises call "evergreen" software, where they don't get involved in approving every update and things are kept silently fresh. With time you'll see discussion of old browsers and what to do about being compatible with old browsers die out.
Straightforward login API: that's OAuth. The idea is that the login is always done by the end user, the human, and that the UI flow is always directly with the account provider. So if you're a desktop app you have to open an embedded web browser, or open a URL to the login service and intercept the response somehow. Then your app is logged in and can automate things within the bounds set by the APIs. It's a good tradeoff - whilst more painful for developers than just asking for a username/password with custom UI each time, it's a lot more adaptable and secure. It's also easily wrapped up in libraries and OS services, so the pain of interacting with the custom web browser needs be borne by only a small number of devs.