Grimm's law and verner's law

Verner's law (possible boundary for Indo-European/Germanic)
Grimm's law/First Sound Shift in the late 1st century BC (does not mark the formation of Germanic accordingly)
Appearance of initial stress (third possible boundary for Indo-European/Germanic)
This chronological reordering would have far-reaching implications for the shape and development of the Proto-Germanic language. If Verner's law operated before Grimm's law, one would expect the voicing of Proto-Indo-European *p, *t, *k, and *kʷ to produce *b, *d, *g, and *gʷ, which would have been identical with the existing Proto-Indo-European voiced stops. Yet it is clear that consonants affected by Verner's law merged with the descendants of the Proto-Indo-European voiced aspirate stops, not of the plain voiced stops. The usual proposed explanation for this is to postulate aspiration in the voiceless stops of the dialect of Indo-European that gave rise to Proto-Germanic.
Here is a table describing the sequence of changes in this alternative ordering:

PrePG	*pʰ		*tʰ		*kʰ		*kʷʰ		*s
Verner	*pʰ	*bʱ	*tʰ	*dʱ	*kʰ	*ɡʱ	*kʷʰ	*ɡʷʱ	*s	*z
Grimm	*ɸ	*β	*θ	*ð	*x	*ɣ	*xʷ	*ɣʷ

(This can however be bypassed in the glottalic theory framework, where the voiced aspirate stops are replaced with plain voiced stops, and plain voiced stops with glottalized stops.)
Having Verner's law act before Grimm's law may help explain the little evidence that there is for the earliest Germanic phonology. There is some evidence for dating Grimm's law changes only to the end of the first century BCE. In particular, the tribal name recorded as Cimbri by the Romans and the river-name recorded as Vaculus (now known as the Waal) suggest that the change from k to h had still not happened around the first century BCE, when Romans were rendering those words into Latin (unless they were rendering the early Germanic *h (/x/) sound as a /k/ because their own /h/ did not often occur between vowels and was at any rate already in the process of going silent). If Grimm's law was operative only in the first century BCE and Verner's law applied after it (followed in turn by the shift of stress to initial syllables), then three dramatic changes would have had to happen in quick succession. Such a rapid set of language changes seems implausible to some scholars.¹¹
Meanwhile, Noske (2012) argues that Grimm's Law and Verner's Law must have been part of a single bifurcating chain shift.