2. The JSON is pretty standard, I just use jQuery serialize on the forms to POST on your URL. MailChimp won't work right now with our Form Builder, but you can easily embed your own MailChimp forms by selecting "Use My Form Code" on section 2) Form Content. Several users already using OptKit with MailChimp.
3. I say enterprise grade because of the amount of segmentation you can do, already. Just with timing + geolocation limiting, you can segment messages. More features coming soon to strengthen this claim as well.
4. Yup :) You will be able to test any setting with a split test.
5. You can use your own code in both sections 1 and 2, so there is a hack-y way to get some GA codes in there.
That popup experience is obviously not ideal.. And not how its working for me.. Can you tell me what browser you are on?