The reason you don't have much freedom on the kibbutz before the army is because you're paying for your spot by working there. Every program that comes with free housing will of course come with certain restrictions. There's no way around that and I think it's reasonable. You may want to contact them individually, though, to discuss your situation and see if they will allow you to stay for less than the minimum 2 months or to create some other kind of arrangement as a soon-to-be-enlisted volunteer. They may have a financial incentive to do so if you stay at the kibbutz during your service and they receive your housing money.
I'm not sure what you mean by "which date you are chosen." Do you mean if you have a say in your enlistment date? The answer to that would be yes, more or less. First, you still haven't answered my question about the registration form. That should be your first step in any case when you're even considering the program. There is no obligation attached, but it will determine your eligibility. Once you have done that, and when you've made your decision, you can go ahead and plan to get the ball rolling to get into a specific draft. The relevant links are all on the main site at
http://www.mahal-idf-volunteers.org . Specific enlistment dates are posted there as they are announced by the IDF. You can prepare for these specific dates and plan your timeline around them. There is no guarantee that it will all work out for a specific date, especially if you haven't followed instructions or obtained all the necessary documentation in advance. However, if you do follow instructions, get the documents you need before you leave, arrive in Israel on time, and do what's required, there's generally no reason you shouldn't start your service at the posted date for which you've planned. However, once you've received your orders to report for your first day of service, you can't simply "reschedule" either to a different draft or to a different day, of course.
Does that answer your questions?