If I want to talk to an animal (in this case, with INT 2 -- 4 is max for the spell) and potentially affect social checks, then this is the spell to use. If the spell succeeds, then the beast is Charmed. If you Charm something/someone, you do get bonuses to social ability checks. The RAW say nothing about a failed attempt being seen as a threat or attack; only that it knows it was Charmed once the duration ends.
As far as the Larian DM--there was no saving throw shown on the Deep Rothé when I cast it. The casting was simply seen as an attack, which is kinda wtf. Perhaps they assume it's a spell only used in combat, but that's not the case. I can toss this part into the suggestions area, though, if Larian is treating a charm spell as an attack when used out-of-combat.