I don't know the rules well and have not played FFT3. I took a quick look at the PDF slimmed down version. Based on these caveats, here are some thoughts FWIIW. The biggest problem (which is generic to WW1 not just 1914) is that the unit frontages are too wide. Typically you can scale the frontage by a factor of one unit compared with WW2 – a WW1 company would occupy the frontage of a WW2 platoon. Straightaway you can see that this addresses the cohesion distance issue.
Rather than mess around with unit size, you might consider using a different ground scale – reducing from 1" = 100 yards to 1" = 33 or perhaps 50 yards. The table size would remain the same but immediately you can see that the table would represent an area about 1/3 the size of a FFT3 table for WW2 or modern war.
In 1914 you have batteries firing directly over open sights or using FOOs that are only a short distance from the batteries. Most commonly, the batteries would be positioned in defilade, such as just below a ridge line. The FOO would be located on the ridge with line of sight to the battery. I would not recommend a fighting stand being able to call in artillery in 1914 but this is a very reasonable mechanic later in WW1. Great War Spearhead uses the same abstraction, which reflects the complex network of systems that enabled WW1 artillery to spot and engage enemy infantry or artillery during a battle. Ground-based FOOs, SOS calls, captive balloons, and aerial spotting all fed into a centralised fire control system. From the enemy perspective, any time a unit tried to move in the open then, unless the enemy artillery was suppressed, the unit would come under fire almost immediately. Even with pre-planned barrages, it was usually the case that up to a third of artillery would remain on standby for called-in fire support missions.
I wouldn't worry too much about the 'poor communications' issue. Too much is made of this. During the Battle of Mons, for example, the BEF II Corps was receiving up to the minute messages about the German advance and breakthrough in the Nimy-Obourg salient. I imagine FFT3 has mechanisms to limit units ability to respond to new orders anyway. Stick to these would be my advice. 1914 battles were not as 'in the dark' as you might imagine.
Robert