I don't know why there are 2, but the google.com/accounts one came first, possibly before Google had public profile pages, and then they couldn't turn the old one off.
The Google OpenID provider returns a unique per-domain key to the website, so they can't tell your Google address from your login. However, this privacy measure leads to trouble when trying to use the same OpenID to identify yourself on multiple related sites on different domains, such as StackExchange, as the keys are all different, they can't be connected.
The Google Profile OpenID provider uses the more traditional, consistent key, approach, but exposes your profile address to the website you are logging into.
In answer to question number 2, no. Your accounts wont be linked automatically. StackOverflow and SuperUser will see them as totally separate logins. You need to use your Google Profile address to make that work.
I don't believe that Google are planning on phasing out one or the other, but if you are worried, I'd stick with the Google Profile OpenID. Don't forget that an OpenID is an OpenID, anywhere that supports one supports another, even if you have to type it in manually instead of clicking a Google logo.
Best Answer
No, not unless Yahoo exposes that information.
It is possible for an OpenID provider to do this, as they do know of the authentications, but if Yahoo doesn't expose it, then .. well, that's that.