1- for me the product approach and the process approach are both very important because i feel that one completes the other ,but if i had to choose i would choose the product approach because i lean more to accuracy and speaking and writing in a very accurate way , after all this is the goal of teaching a language and for me personally i like perfection because it indicates that the learner is doing a hard work and making a serious effort for learning.
2- of course one approach is more preferable than the other in some cases and contexts like if we take teaching children or teaching businessmen for example i would choose the process approach because for children the idea is to make them write their thoughts and organise them in a correct way and help them build their self confidence without interrupting them or make them busy with grammar which can be tricky for some young learners, same for businessmen , this group of people do not need accuracy in their English , they need to communicate with people and they need to learn the language in a shorter time than others, and they can work on their accuracy later. as for teaching English for academic students or preparing for an exam , the product approach flows on the surface because this , according to my opinion , is the more and most appropriate method or at least it should be taught or used before the process approach.
3- other writing activities in my opinion is observing how writers organise their books and novels , of course a beginner writer would have no idea how to write unless they see other people work ,and it is not a bad thing to quote or take some ideas from other writers or study the way a specific writer organise their work and then learn the steps , and i am not talking about coping , it is just that professional writers have some almost perfect ways of teaching writing.