1) Is it okay to change the date that my wife and I met on the VAF4A as I was off on the date by about a week? I just want it to match the actual date, and the one in her letter of support which includes a history of our relationship. Do we just cross it out and put the right date beside it, initial and date it?
Yes, that's fine to change it. Just cross it out and write in the correct date. If you like, you can include a note in the cover letter to say why it has been corrected.
2) Where do I need to write the IHS reference number on the visa application?
Wherever they can see it. You could put it at the top of the first page, maybe.
Also, if you print out your email receipt with the number on it, you can include that as well.
3) Is it okay if I only buy return shipping as my wife took everything we had back over to the UK and she will be posting it from there?
Yes, that's fine... though how are you getting your passport, biometrics receipt and application forms to her?
4) Do I need to write any descriptions by the screenshots in the WhatsApp call/text log?
Nope.
5) Do I need to write any desriptions for the printed photos of my wife and I?
Nope. You should only have 2 photos anyway - one from the wedding and one from another time in your relationship. All they need to show is that you have met in person... doesn't matter when or where.
6) What does my wife need to put as our reason for choosing to live in the UK (which was noted as a requirement for her letter of support)?
You put whatever your reason for choosing to live in the UK is.
We can't tell you what to write, because we don't know what your reason is.