The majority of the IPA you find at the store is going to be anywhere from 9-30% water regardless, and there's almost no ferrous material inside of a membrane keyboard (aside from the guides typically used on Shift/Return/Space/Caps Lock/Tab keys). The oxidation process of Iron occurs at a very low rate unless it's in the presence of an electrolyte such as salt...and you're not really providing much of an electrolyte aside from any residue left on the keyboard; which after rinsing/dilution with even just a liter of Distilled/DI water will result in an extremely negligible concentration. Far more likely for IPA to attack the various polymers a keyboard is constructed from than for a rinse in distilled water to cause the very few components made of ferrous material to rust. The vast majority of the water will have evaporated within the course of an hour or two anyhow...concerns of oxidation are really only founded under prolonged periods of time in a high humidity environment...not a single rinse.
Firstly you're not correct about the oxidation of iron, as it will oxidise readily with water. Yes, it will happen faster at lower ph and with electrolytes, but it will happen readily with water.
You're also incorrect about IPA and plastics. In fact most commercial keyboard and screen cleaners are IPA. For many plastics such as nylon, polyester, ABS, and especially solvent- sensitive polycarbonate (Lexan), isopropyl alcohol is recommended for cleaning purposes. In fact, because of polycarbonate's high solvent sensitivity, the only universally recommended cleaning solvent for this plastic is isopropyl alcohol. The only real exception is vinyl.
Id rather take the very limited possibility that any of the plasticisers diminish rather than putting water on my electronics any day as PET, HDPE, PVC, LDPE, PP,PS and PC are have very low solubility in isopropanol and even their plasticisers require prolonged exposure.
Essentially my point is, water (even dionised) will rust metal parts, whereas IPA is very very unlikely to affect plastics.