As you said our realities are different. Since you work at an agency there is a constant push and pull between your clients and the project managers to establish some kind of deadline. Projects should be more 'clear cut' and you should be doing less of the kind of experimentation that helpes evolve products iteratively. So to answer your second question first, there should be a big historical database of successful projects that have been delivered at your company that are the 'bread and butter’. If there is no written record of this it should still exist in the minds of the people who have done this kind of project planning at your company before. This should be your starting point and you should be able to structure projects at a high level and give a realistic delivery date based on the project history at your company. Of course you should take into account as many variables as you can and often times this means adding some measure of ‘slack’ that gives you comfort that you are planning taking into account things that could happen that are outside your sphere of control.
To answer your second question, I see points as a kind of euphemism for hours. Developers don’t like to commit up front and this is of course acceptable when there are no formal set of requirements. Since you work at an agency requirements should be better defined from the start. This doesn’t mean planning everything up front but it does mean that you as a project/product manager have to invest more work into structuring out better defined tasks and having a more efficient continuous grooming process that has a lot of client involvement. So in your case I would drop the 'points' and work directly with hours. You shouldn’t assume initial developer estimations as rock solid commitments. You should use their estimations to help you have a better view of how on track you are regarding the delivery dates that you have established with your client. These hard delivery dates are problematic but there are many ways to better deal with hard delivery dates and any strategy you use is better served by having the client as involved as possible in your processes.
Thank you for your comments and I hope I was able to help out a bit with my answers to your doubts.