How long will a 12GX210F carry a 25 amp load?

Past eight hours to 1.75 volts per cell at 77°F, where the published figure is 26.2 amps. At ten hours the block gives 21.3 amps, so a steady 25 amp draw lands between those two columns. Raise the end point to 1.85 and the eight-hour figure falls to 24.8 amps, which puts the same load just inside eight hours. The disconnect setting decides which of those answers applies to your site.

Why are there two different resistance figures for the 12GX210F?

Because they are measured differently and used differently. The specification sheet gives approximately 2.9 milliohms internal resistance for a fully charged block. The PWRSS ohmic readings guide gives instrument-specific reference values for a new block: 2.81 milliohms on a HIOKI 3554, 3.15 on a Biddle BITE 3, 1,732 siemens on a Midtronics PBT-200, with initial readings varying up to 25 percent. Record your baseline with the instrument you will keep using, and compare later readings only against that same method.

Does the 12GX210F need a deeper shelf than the smaller GX blocks?

Deeper than the 100F, the same as the 190F. The 210F is 22.0 inches deep and 4.92 wide against the 100F's 16.0 by 4.25, and it stands 12.9 inches tall against 11.3. Against the 190F the footprint is identical and the difference is 0.3 inches of height and 6.4 lbs. Measure front to back first on any cabinet originally built for a 100 Ah class block.

Can a 12GX210F go into a string of 12GX190F blocks?

Not as a partial replacement. A series string discharges through every block equally, so the capacity available is set by the smallest unit in it, and mixing a 210 Ah block into a 190 Ah string buys nothing while complicating the ohmic baseline. Power Storage Solutions publishes no mixed-capacity guidance either way. Replace a string as a set and keep the capacities matched; if the target is more reserve, move the whole string to the 210F.