...
What would happen if Germany, instead, reversed this plan? What would happen if they simply held their own border with France and instead worked with the Austro-Hungarians to launch an offensive against the Russians first?
Well, this is (almost ?) a/the classic AH-trope : the "East First"-version of WW 1.
... and though often and heavily discussed within the Alternate History community (not only on this site), there are not so many (at least known to me) attempts to actually write such an ATL.
Perhaps the 'most known' is
"Grey Tide" with it follow ups.
And this is also rather 'short' on the practical side, as how the germans actual conduct their 'alternate' eastern campaign.
Most of the time the discussions 'work' with the last fully and truly by the german general staff prepared plan for such a campaign : the "Aufmarsch Ost II" for the mobilization period of 1912/1913, planned and compiled in 1911.
For the mobilization period of 1913/1914 there was only a 'study' (much abbreviated 'plan')for such an eastern campaign, that - as far as can be said today - didn't differed much from the 1912/1913 plan. ... especially regarding strategic-operational 'ideas' as the gigantic
battle-of-Leuctra or maybe better for a prussian-german reader ;-)
battle-of-Leuthen idea the Schlieffen-plan actuall was.
(
The sooo often named battle-of-Cannae trope is only the result of generations of uneducated copy-n-paste. It was - if at all - only an operational sub-plan of what Moltke made out of Schlieffen regarding the fight in Lorrain.)
However. This plan made-up by Moltke the Minor in 1911 was uninspired, simple-stupid (or simply stupid) and in now-way up to the task. In essence it was a front against front shoving of forces up to the russian fortress line and then ... sry but ... no idea at all.
... as Moltke the Minor also had IOTL in 1914 no idea at all, what to do against the russians, than defend as envisaged and planned by Schlieffen and the longstanting army-inspector of Eastern Prussia who - as inspector of pioneers and fortresses forces - planned andconceived the fieldfortications in eastern prussia present in 1914 as well as induced the operational training for this case : Colmar v.d.Goltz.
But for your further questions :
Would they be able to keep Britain out of the war?
Consensus (more or less) on this board is :
not for the whole time of war.
At some point GB would - still - join the frail against the germans, rather earlier than later. ... though (also still) noone ever provided me a reasonable, sensible, possible as well as working and probably other casus belli to draw Britain in than the belgian violation (beside some hand-waving and nebulous 'they/press-will-fetch/invent-some').
Would they be able to prevent the Italians from siding with the Entente?
As long as GB also stayes out ... IMO of course.
After the first slaughter/bleeding-white of the french running against german defences in Lorrain GB might even need some ... effort keeping the italians fom joining for some mussoliniesque french-bashing. But as soon as Britain enters against the CP ... Don't see why the italian politicians wouldn't be as stupid as they were in 1915 IOTL.
Would France attempt their own offensive into Germany?
YES !!!
They were obliged to do so on the terms of their alliance with Russia to attack at the 14th day of mobilization at last, parallel to the russians.
They were obliged to do so on their own revanchism as well.
IOTL in the nigth from 1st to 2nd August the french prime minister Viviani as well as the french president Poincaré told the russian ambassador Isvolsky being informed on the german 'DoW', that France would OFC fulfill its obligations, but that due to political domestical issues they would rather wait for their DoW against Germany until the french mobilization has been finished at about the 12th August. Also, that they would render it much more advantageous (for again domestistic but as well international reasons) if the germans would do the "first step" and start the attacking ... against France. ... A 'favor' IOTL the germans did.