No, you don't. You *assume* that it is equally distributed and you *assume* that it is increasing at the same rate. More assumptions than you can shake a stick at.
And yes, the 34 billion year examples do not 'form an isochron' because they are thrown out. The reason that they are thrown out is because they do not form an isochron (a linear distribution).
Circular reasoning and truth by definition.
I've already dealt with this. Entropy causes isotopes to be evenly distributed. We need specific fractionation processes to differentially concentrate isotopes and reverse entropic effects. Fractionation effects are known to be very weak for heavy atoms, and known to be very weak for spherical divalent cations like Sr2+. You can dance and wave and misrepresent these as assumptions, but they are not.
Strontium isotope ratios have been extensively measured for many parts of the planet, and except for the small correction due to the age of bedrock, they are well-mixed.
And yes, the 34 billion year examples do not 'form an isochron' because they are thrown out. The reason that they are thrown out is because they do not form an isochron (a linear distribution).
They are highly scattered. We know of processes that will cause them to scatter: leaching of rubidium is one. We know that in the absence of such processes (and a few minor effects discussed by Zheng) they will give an isochron. And we know other samples from the Pahrump formation give a consistent age of 1.2 billion years.