Warren’s GEs for non-engineers are fairly in line with the GEs at the other five colleges. (Personally I think its engineering GE load is ridiculously light, possibly excessively light since the whole point of GEs is to give you a more rounded education. Some of my fellow engineers could use one.) Warren Writing is supposed to be easy, but kind of boring. If you’re concerned about the workload, you could try to take one of the “easier” or “more fun” PofCs that still interests you. Don’t pick one just because it’s easy, because you’ll probably struggle with its upper division coursework if you do. If you’re not a fan of taking a lot of math, a math major might not be for you…
You can take most upper division CS courses. They may say “restricted to major” or “departmental stamp required” but if you shoot the professor an email expressing an interest in his/her class they’re usually very helpful. Upper division courses usually have space, I’d be more worried about getting into the lower division CSE classes which give priority to CSE undergraduates.
I’ve heard rumors that the Data Science major was approved by the academic senate but might not be available until 2017 or 2018. I don’t think it will actually change the CS landscape that much, because with all the people trying to major in CS-related fields a brand new major will reach capacity pretty quickly. A lot of students may also see it as too specific for their liking, or they might actually be thrilled about its more interdisciplinary focus. Your guess is as good as mine.