Yes, you can hire an appraiser. If you're buying with leverage, the lender will almost certainly require an appraisal to establish the as-is value and the ARV before closing. You can get a good idea of the ARV based on research, but your lending options will be tied to the appraiser's value. I initially evaluate a deal based on the agent's CMA, but you can be screwed in the end if the lender's appraiser thinks differently.
No, property does not need to be 100% vacant to BRRRR. If looking at an SFR, I want it to be vacant so we can immediately start rehab and reduce the time our money is "stuck" in the property. Inherited tenants just seem to go badly. For anything with more units, you could always use income from other units to help defray the holding costs while you rehab and reposition the property, and cycle through the units. There's no right or wrong answer to this, it's more specific to the strategy that best fits the property and your goals.
If you're using financing, then the "true ARV" is whatever the lender's appraiser says, because you're dependent on that to ultimately do your refi. Keep in mind if you use financing to purchase the property, and then refi again, you take the risk that the second appraisal will not match up with the first.