The whole question of the endings of the ablative and instrumental has been reexamined in a far-reaching article by H. Craig Melchert and Norbert Oettinger, Ablativ und Instrumental im Hethitischen und Indogermanischen. Ein Beitrag zur relativen Chronologie. The findings and suggestions of this article are extensive, but I should mention at least two.
On pg. 207 fn. 7 I mention the idea that the PBS instr. pl. ending *-mīs might come from *-mins with an n imported from the accusative. But this was never too likely. Melchert and Oettinger instead suggest that *-mīs was modelled on an unattested but reconstructable *-bhīs < *-bhi-is.
On pg. 212 I compare the Hittite instrumental suffix -it with the Vedic adverbial suffix -it. But Melchert and Oettinger argue that the underlying form of the suffix is -d as in the pronominal form ke-e-et ÍD-az /kēd hābats/ 'on this side of the river' etc. and that the i is epenthetic. Hence there is no comparison between Hittite and Vedic.