You need to prove you were physically present in the UK exactly 3 years before the date of application. Don't count on your passport being accepted as proof of residence. Most people provide P60s over the 5 years which show they were working for a UK employer, letter a from their employer, some proof of continuous residence like annual council tax statements.
No, of course referee statements don't need to be notarised, they are personal statements. It's fine to use the same referee declaration for both
Your BRP shows you are free from time restrictions. Yes, your passport and BRP will be proof for multiple sections here
I am not an immigration adviser and my opinions should not be construed as legal advice. Please do not PM me for advice.