Appointment + Back end Double bookings Allowed

So during 2½ year on wpmudev I have made about 10 wordpress sites that requeired a booking system. During that time span I have used Appointment+ ONLY ONES as I find it always comes to short in features and UI of this millenium. Actually twice because I am in the progress of using it again and this time I realise another big developtment F#&k up.

I am almost done with the project and now I realise
------- When creating a booking via the back end, there is no check that hinders double bookings ---- REALLY?

Imagine a hairdresser. She gets bookings via her website, GREAT it works fine thumbs up for appointment+++

But some customer send her an email, message on facebook or call her. She goes in the back end and create that booking manually, but there is a risk she will create a DOUBLE BOOKING.
THen next tuesday 2PM both Tyra Banks and Heidi Klum comes to the hairsalon to dye their hair. THe hairdresser has only 1 chair and 2 arms, Imagine the drama!!!!

I see 3 solutions
1. The hairdresser need to look at the front end calendar and tab back to booking page to create the booking. Very not user friendly and most normal people dont know how to tab between websites.

2. I ask the hairdresser to make a google calendar and she needs to put the manual bookings in google calendar and we use the 2 way sync it will avoid double bookings. Oh and the client dont get an confirmation email, she need to then go to appointment+ and send an confirmation email.

3. WPMUDEV develop a setting "allow double booking in back end "Tick box YES/NO" if no
- THen creating appointment in back end will be same as now, except the time and date field will be replaced with the front end calendar clearly displaying free spots and leave no way of double bookings.

4. as the wpmudev support worker said, I could develop the function myself. Eeh no thats not an option, then I can pay the 99 USD for birchpress that will work instantly.

Any other ideas?