The issue I see with the best two out of three and insta nom option is that combat vore would go from being underpowered to too strong. Like Taizee will eventually get the bite attack and chomp, and with these rules she would just be able to bite and nom on AoE, and when she gets multiple attacks a turn she would be able to nom multiple enemies a turn. This would kinda just lead character building to be an armsrace between grapple CMB and CMD, as well as stomach AC, with everything else just becoming sub-par