I think it's really up to you who you have at your birth. I wouldn't worry about how other people feel about it too much. It might be wise though, to tell some of these people that you *might* decide not to call them, depending on how you feel. It could well be that you'll get into labour and really feel the need to be private and alone, and if you've got a crowd of people there, that could be difficult. But if you feel like you want a nice group of people there with you supporting you and cheering you on, then you'll have plenty to call on!
I would maybe suggest that there be someone who can care for your son, who won't mind missing the birth, if necessary. Your mum might be a good choice for that. He might be upset, or just bored, or whatever, and he'll also need his usual care - snacks, meals, etc., and someone who you really need with you can't be running off to do all that for him.
About the doula - I haven't had one myself, but I've heard that midwives generally like to have a doula there. Her focus is different than the midwives'. They are going to be looking a little more at your physical wellbeing and safety, as well as the baby's, whereas the doula is focused more on your comfort and emotional state. There is definitely overlap - a good midwife will also be concerned about your comfort and emotions too, obviously, but they might not be her primary concern during the birth.
All the best with getting the final details together!
Oh - about the scents - more choices are always better I think. You never know what you're going to want in the moment.