Firstly, I would get and idea of what your minimum living expenses really are. You say you'd love to work a few days a week in a cafe or book shop, but worry that it would not pay well enough -- you should figure out how much you need so you know. You may need to adjust your lifestyle - it is far easier to not spend a dollar than it is to get a dollar.
You may find it hard to get a part-time job to start off with. It may be easier to work a full time job for a few months, and then ask to cut back. If you save money in the mean time, you can quit if they refuse and seek out a better position.
There are a lot of posts here about the best kinds of part-time jobs. In reality, you are probably not going to be able to pick out the ultimate pay-the-rent-while-I-code-my-startup job; you are going to end up with one of the first few you actually find, and finding that job itself may be a full time effort for a while.
In that light, I'd like to suggest that the actual job and environment doesn't matter so much as you'd think, and that the fact that it is DIFFERENT is what matters.
Ideas for good products come from experience, not from cloistered ivory tower environments. You might come with a great product after working with several different inventory or point-of-sale systems, or observing how an independent roofing contractor maintains all his contacts on his phone, or whatever.
All the great American writers first learned by writing about real life things - Steinbeck filed many short newspaper stories on dust bowl refugees before he wrote the Grapes of Wrath, Clemens did the same and wrote of real experience in "Roughing It" before his masterpieces.
Here's my advice: 1) know your expences and cut them where appropriate; 2) get a job, and quit it within the first 2 weeks if it obviously sucks, and then get another one; 3) while working, agressively attend meetups and similar events so you don't become socially isolated; 4) after 3 months, ask your boss if you can cut back to part time if you need to do that - if he says no, work another 3 months and ask again; 5) if he still says no, quit, re-evaluate working a corporate development job, and start over again at step 1 if that's what you need to do.