So I've looked & can't find an answer, don't know maybe I is a dumb, but do add the abilities mod from both templates. (i.e. A half-fiend gains a +4 bonus on three ability scores of its choice and a +2 bonus on the other three & A half-balor gains Str +6, Dex +4, Con +6, Int +4, Wis +4, and Cha +4.) If so sound like i got a villain to punish the party a little.
You don't add them together. Half Balor just gets the specific ability score bonuses.
K, wasn't sure as unlike the rest of the changes, it didn't say "replace with ..."
You use the Half Balor ones. There is a reason the CR bonus is greater vs. the stock Half fiend.