Regarding your hen do - why don't you organise a spa day for you, your sister, and your bridesmaid with ME? You can get some really good deals for numbers like that, and it could be a really lovely way to relax before the wedding.
Lots of deals offer afternoon tea, or lunches with fizz. Treat yourself and your girls who have stuck by you.
If you wanted, you could always go out for a meal and cocktails in the evening, or have a girly sleepover with pizza and facepacks!
Regarding your actual wedding, is your OH helping out? If not, get him involved!
Getting married at a zoo is so cool! Are you able to get into the rooms the day before to set your tables up? And maybe speak to the zoo, see if they can let you have anyone to assist with this - or maybe your OH's family can travel down the day before to help with venue set up? Then you have nothing to worry about on the day.
If you're using reputable suppliers, they should be professional enough to not need to check in with you over every little thing on the day, see if there is a contact at the venue they can use.
Also, whilst I can see it could be frustrating not having lots of people helping you organise, it can also be a blessing - too many cooks spoil the broth as they say, at least you get to have the wedding you both want without a zillion people giving their opinions and making matters confusing, so try to think of it that way.
I hope this helps, it sounds like you're wedding will be wonderful! X