Karina, I agree with the others that you should give them notice now that you intend taking the 12 months, and then decide down the track. You may get to 10 months and decide there's no way you're going back, or get to 6 months and think if you don't go back full time you'll go crazy.

In relation to them changing their minds on part time/working from home - don't give up hope just yet that it won't happen. If you are the first person in the company to be in this position the thought of working from home, part time, job sharing etc may all be too daunting for them, so they've opted for the easy "full time or nothing". When the time comes to make the decision about going back, raise it again. They may have realised by then that you part time is better than no you at all! Good luck with it.