Richard,
• Can you please confirm our Current running ARC Enterprise Premium v5.1.1 is now called ARC Premium v5.1.1?
Indeed it is the same, it was just rebranded. In fact, in V6 it is now called Arc Pro
• option1: From ARC Premium v5.1.1 upgrade to Arc Premium AC 5.1(4) then to the latest ARC Premium 6.0.0?
You would need to speak to Arc (maybe via a partner if one was used for the purchase) for advise on upgrading Arc
• option2: From ARC Premium v5.1.1 upgrade to the latest Cisco CUAC Premium or Enterprise v8.x?
You cannot upgrade Arc to CUAC without making a new purchase and doing a new installation