Concerning your second point, as Durk said you can adjust AI settings in the Options menu on the main page. Some things that might help are setting the AI difficulty higher, raising the AI's aggressiveness, and giving the AI activation bonuses. There are some War Plans the Central Powers can take that delay the UK joining, so that can add to the challenge as well; but there's no way to make the AI choose those plans.
Once you're comfortable with game, to make things more interesting and to give the AI a leg up, you can put restrictions on yourself. As the French, you could be committed to the offensive early in the war, as they were historically. Just an example, and it may not be your thing.
Hope you enjoy the game. These forums are a great resource