Pentcho,
Slowing light produces blue shift. It does so in a medium of higher refractive index, which equates to Polarisation Mode Dispersion (PMD) which slows the bulk propagation speed due to interaction density.
A GRIN lens shows this by having an effective graded density, producing curved light paths and non-linear blue shift. When light re-emerges from the denser medium it reverts to local c and reverts (redshifts) back to it's original wavelength. All this is irrefutably proved in optical science experimentation.
I agree cosmic redshift does not prove accelerated expansion, but you have again wrongly identified the cause of the redshift. That does not aid the task of getting physics back on track so more care is needed not to give dissent a worse reputation than it already has.
Consider; If space with a wave embedded within it expands, what might you expect to happen to the wavelength? In an intelligent approach accelerated expansion can be shown to be self defeating.
Best wishes
Peter