You may be looking at the upgraded "all-access" pass. No need for that. The 4-day Keystone pass on their site, for early Feb., shows at $340. And it does allow 1 day at Vail. If you don't believe me, call VR :) (I did, twice. Emailed them too.)
I did write "of course any Keystone days could be spent at A-bay." I'm not focusing on A-Basin because it's no cheaper than Loveland, and at 900 acres, it's also half the size of Loveland and a quarter of Keystone. Even more importantly, I'm trying to narrow down my choices, not expand them :) I have nothing against A-Basin, or Monarch, Eldora, or Crested Butte for that matter - I've never skied any of them. Just trying to make some good choices for my first-ever CO ski trip based on my skill level, income, and personal tastes.
1. You're right -- very date dependent. I selected dates slightly later in feburary. I'm not seeing any mention of Vail, just that Breck can be split 50/50 on passes over 3 days or more, unless you upgrade. Maybe it's allowed and just not stated on the website.
2. A-Basin doesn't ski 1/2 the size of Loveland. Loveland is a bit inflated because a few hundred acres are in the Valley (which you won't ski) and most of it is off the Ridge. Which depending on your skill/timing may not be accessible. I'm a season passholder at Love but am under no illusions about it really being 2 times the size of A-bay on anything but paper.