Hi Aussiebird,
Yes, your Australian stat decs need to be certified and a photocopy of passport also needs to be certified. The same person certifying the forms can also certify the passport photocopy
For the two from the UK they don’t need to use form 888, instead they can just write a stat dec that is legal in the UK. There are lots of examples on the internet of what one in the UK should look like. Ensure they put contact details of a phone or email on the stat dec. Yes, a certified copy of their passport is probably needed.
We had a few people tell us that 6-8 is a good number of form 888/stat decs to have, are you able to have a few more written?
Also yes, applicant and partner need to upload passport photos, I would think they need to be proper passport style and not just photo booth ones.
Have you considered using an agent? We did and it took a lot of stress out of the whole application. Sure it adds to the cost but the thought of not getting a visa by missing something silly ourselves seemed cheaper in the long run!