1. Probably because the CMRR degrades with higher input voltages, something they don't want to tout. Given your accuracy requirement, don't care.
2. No, less accurate. It's the same opamp and presumably the same tolerance resistors. The difference between 1K and 1M input impedances to a 0.1 ohm source impedance is trivial, and overpowered by the input offset voltage errors of two additional amplifiers. The only instrumentation amplifier designs that are a measurable improvement over a well-done diff amp design are the ones that use fully integrated, laser-trimmed instrumentation amp ICs.
3. With a 10% error budget, an LM358 would work in this application.
ak