for one France can forget to buy stuff in the US. OTL the Allies received some 3 billion dollars of goods (roughly 700 billion in todays currency) and Germany because of blockade only 25 million (0.6 billion) from the US.
If France violates Belgian neutrality the US will scale back its trade with it tremendously and instead trade with Germany. The French fleet is not strong enough to enforce a continental blockade especially not against the wishes of Britain. So Germany fights only France and can receive products from the US = overkill
And as I have allready written - on the defensive the Germans can bleed the French dry - especially when the French will have to sustain massive supply lines not on ttheir territory - something they never had to OTL.
Maybe these points are hindsight, though.
If Joffre launches his attack through southern Belgium, I figure it would be immediately after his armies in A-L get held up. The lesson he will draw from that is "If they're so strong in the south and center, their right must be weak". He will strike immediately in order to take advantage of the "weakness".
So, if a French Ardennes offensive is launched, it will still be very early in the war, before the delusions have been dispelled.
And since one of the main French delusions was that the war could only last a few months, any potential impact on trade with the U.S. would be considered irrelevant -- the war "won't last long enough" for it to matter.
And of course, in this early war period, the French still thought that French elan + French 75s can beat anything the Germans might do.
In the aftermath of defeats in A-L, Joffre's assessment was that some commanders hadn't been determined enough, and the 75s hadn't been used profligately enough in supporting the attacks. It wasn't until well after September that he accepted that his doctrine had been hopelessly flawed. So your point about the Germans bleeding the French dry, is valid, but Joffre won't see it until much later.
So I agree with your points, but they seem unlikely to have any effect on French decision-making.