Understanding the new application process

Hi everyone,
Currently on tier 2 and switching with my fiancé who I have been living with for nearly two years now. We are planning on applying at the end of January. Two questions:
1) Does anyone know how early you can pay for the IHS surcharge? If the increase comes in to effect on 19 Dec and I apply around 25th January will I be OK?
2) We were/ are planning on doing the 24 hour decision service which I understand used to be available. I find the whole process of applying for visas incredibly stressful (I know I'm preaching to the choir here) so it would help me if someone could explain what exactly happens from the time you click submit on the online application form to actually attending the appointment. Do you actually get an appointment within 24 hours of submitting your application? Does your partner attend the appointment with you? How long is the appointment? How does it all work with these new centres? From what I can read it just seems like an extra £100 for them to scan your documents for you!

Thanks for your help.