The demons weren't on par with the goddesses when Meliodas left, they were weaker, that was the whole point of the GCs attack, because they had the advantage. Meliodas maintained the balance, and when he betrayed the demons the balance broke, so Gowther took someone equal to Meliodas', Mael, and swapped him over, using him to bring back the balance Meliodas once upheld. The GC wanted a secure victory, not a drawn out war, because they don't like casualties, so they went for a draw instead.
This is all said by Gowther, I'm barely even paraphrasing, so it's all canon.
Oh and Indura that weren't commandments are far weaker, so they shouldn't be a threat as long as Elizabeth is around. It's only the Indura of Favor she has to worry about.