How can scientists be confident of isotope ratios in past eras and varied locations, used for radiometric dating?
"Doctor! It hurts when I do this! «bonk»" "So don't do that then!"

One way around the issue of an unknown initial distribution of isotopes is to not do that. One of the most reliable mechanisms for dating the Earth is uranium-lead dating of zircons. Uranium can take the place of zirconium in the zircon crystal because of the chemical similarity of uranium is and zirconium. Lead on the other hand is very dissimilar chemically from zirconium and uranium. The crystal formation process strongly rejects lead. At the time a zirconium crystal forms it will contain no lead. All the lead in zircons comes from the decay of uranium. Two decay chains lead to lead, with 235U eventually decaying to 207Pb and 238U eventually decaying to 206Pb. Those two decay chains provide two independent mechanisms to date the same crystal.

Another approach is to find that initial distribution of isotopes. This is the approach Claire Patterson used in his estimate for the age of the Earth. See Claire Patterson, "Age of meteorites and the earth", Geochim Cosmochim Ac 10 (1956). He used a non-radioactive iron meteorites to establish the primordial distribution of isotopes of lead. With this as a baseline, he then used the disequilibrium ratios in meteorites to find the age of the Earth.

With regard to varying 234U/238U ratios, that's a red herring used by some who don't like the results of radioactive dating in a failed attempt to throw suspicion at those dates. The 256 thousand year half life of 234U means that there is no primordial 234U on the Earth. All of the 234U we see now came from the decay of 238U, via 238U→234Th→234Pa→234U. Those decays weaken rock at the site of the decays. When the rock weathers and is exposed to water, that weakened rock around a 234U atom means that 234U is preferentially leached from the rock.