And what treaty would that be? The French and Russian general staffs held talks as to how to handle what they correctly considered the most likely scenario-. A German thrust through Belgium.
That Russia also had case G which called for a rearward deployment puts your argument in the trash can. Russia kept her options open untill M9 and wouldn't leave her forward bases until M15. At anytime, the Tsar could change the orders.
Only Germany considered mobilization to mean war because her strategy alone depended on speed.
Adding that besides the plan being to attack ASAP if the french adopted the course you propose the russians would surely take kindly to France sitting back and waiting while they are being destroyed by the germans in Poland.
On this point, the main problem with the German “Grosser Ostaufmarsch” (Which Moltke understood at the time) was how underdeveloped the Eastern railway networks were. There was only one useable high capacity double track line running across the Vistula (Marienburg-Koenigsburg). The other line running through Goßlershausen (Now Jabłonowo Pomorskie) wouldn’t have been used because it ran too close to the border, while the single track lines were far too underdeveloped to be of help.
This meant that that the deployment of 40+ divisions to East Prussia would depend on a railway line capable of maximum 72 trains a day (3 trains an hour) assuming it ran 24/7 without stopping. Which of course isn’t how Germany ran its railways, so real capacity was less than the theoretical maximum.
Per
Das deutsche Feldeisenbahnwesen, a regular corps needed 140 trains, a reserve corps 85 trains, and a cavalry division 30 trains. Germany would deploy 20 corps across the Vistula (I Corps was already in East Prussia), so for 12 regular, 8 reserve corps, and 7 cavalry divisions Germany would need 2,570 train movements minimum crossing the Vistula. At the maximum possible rate of movement that’s 35 days to fully deploy in East Prussia, which still excludes miscellaneous, Army HQs, supplies, etc which add a few extra days.
Moltke was well aware that the Eastern deployment plan would take too long, especially with the modernization of Russia’s rail network. Several alternatives were discussed but ultimately dropped in favor of focusing on France. The 1912 Aufmarsch II East acknowledges that 4th Army (Guards, Guards Reserve Corps, X, III Reserve Corps, Guards
Cavalry Division, and 4th Cavalry Division) at minimum will deploy late before/during the start of hostilities.
Other proposals suggested deploying 1-2 armies (1st and/or 2nd) on the German right wing West of the Vistula and having them march into position, which would still be slow and wear out the troops as they deployed. Alternatively, forces in East Prussia could be cut down to 3 armies with a German army attacking from Silesia instead.
So regardless, the minimum acceptable deployment by rail would be the German left wing (Which couldn’t march into position). That’s 4 regular corps, 5 reserve corps, and 4 cavalry divisions, 1,105 trains total. Working at max capacity that would cut deployment time down to 15 days minimum, but this isn’t logistically feasible and still doesn’t include other transportation so we’re probably looking at a completed deployment closer to M+25 days (August 26).
That’s a substantial improvement, but still carries all kinds of risks. If the Russians launched a spoiling attack while the German left is still deploying (Which they could do while still having much longer to mobilize than IOTL), it would throw the entire German plan into chaos and court disaster, particularly if they used all of 1st and 2nd Army rather than splitting them. 18 Russian divisions + 1 brigade and 11 second line divisions with extra prep time attacking 8 regular and 10 reserve German divisions still in the process of deploying would be a nail biter of a battle!
You can see why the offensively-minded Germans decided to forego such a plan rather than risk inviting a serious early reverse.
Anyway, the conclusion is that if the Russians remain on the defensive until the Germans advance as the Russian Plan G proposed they likely won’t be in combat until August 25-30 minimum. So the French have plenty of time to deploy and think about their options compared to OTL.