Is telomere shortening consistant over consecutive cell divisions from zygote to a differentiated cell?
It is not consistent, and so cannot function as a temporal or generational signal. Not because the telomere shortening is unpredictable, but because of telomerase activity in the embryo.

If it did, gamete production in specialized tissues like the ovaries or testes would have shorter telomeres than the zygote they originated from. Each generation would have shorter telomeres than their parents. They would be genetically 'older', which can't go on forever.

Development is incredibly complicated and mediated by a large number of genes. Try homeobox genes for developmental layout.