A simple question? (with a long back story)... What is the best mortar to use to reset a quartzite stone sidewalk? Also, what would be the best material to use for the grout lines?

The back story (with photos). When the home was purchased 4 years ago the stones were in pretty good shape. However, every year more of the grout gets washed away, and it takes it's hardest toll over the winter (pictures attached). Right now, most of the sidewalk stones are loose with most of the grout has washed away. FYI - about 2.5 inches under the top of the stone is a concrete base.

There is a awning that protects the higher portions of the stairs from the deterioration. I've had a few sidewalk installers and masons out for a quote. The job is too small for most to take on. Well, I did have one quote for $2,700 & I thought that was a tad expensive for 60sf of repairs.

The most informative post on the GardenWeb was "flagstone patio vs. mortar vs. sand gravel". Much of the advice was aimed towards a new installation. Since I have a concrete base under the sidewalk & stairs, there is no option for having a sand base.

I'm confident that I can complete this job (will likely take me three times longer than a professional). I've installed marble flooring in my home & a brick paver patio. I have the tools required for mixing and working with mortar. Calling in a couple friends to help with moving the stones.

So, looking for advice on what mortar to set the stones, and what would be the best material for the grout lines (with Chicago's freeze/thaw cycles)? I just want to make sure I use the best materials for the job. Thanks