Does the alabaster color get dirty? Is it washable?

I guess that depends on how much it's used. I have another bag that is that color and it isn't used every day and it has stayed clean. I do think they could be washed by hand. I've put some in soapy water in the sink and then let them air dry and they've come out fine. I wouldn't put it in the washing machine though. It is a great bag overall and I think you will really like it. So far I have 2 of that style.

What is the size of the cell phone pocket? I have a large phone--6.5 X 3.2 inches.

I would say it could. The zipper is a smidge over 6 inches, but there is at least an inch or more inside on the back pocket and a little more than an inch on the inside of the front pocket. It's sure is worth a try and it sure would fit in the bigger front pocket. I love the bag and now have about 6 of them.

Does an MacBook Air fit in it?

I don't think a MacBook Air will fit into it. The zipper is about 11 inches and the inside of the bag is a smidge smaller. There is a larger "Gabbie" and it may fit in that.