Grind a quartz counter top or call a Pro?
To answer your question, “Can a novice ..do it in place”.  Yes you could, but I would NOT.  The hole for the sink is normally cut out before the countertop is installed, with water flowing on the quartz as it is being cut.  All the mess is made outside.  I just had this done recently. 
I have done major DIY project, consider myself capable with a power handsaw.  I just cut up 4 leftover quartz countertops for an outdoor cabinet.  Since this in your kitchen, you would have to use a dry cut diamond blade, and it will be dusty as h…  I believe it will be difficult to cut the parallel line next to the backsplash and a saw guides will be difficult to clamp on the countertop except for the front edge. 

A grinder would not have any support during your cutting session and would likely bind in the kerf as you proceed.  With the power handsaw, I was able to use a guide to keep the blade straight and on the cut line.
I believe the Pro would have better tools and blades to do this cut on an installed countertop and the router bit to do the corners of the sink cutout.  

Again, I would NOT attempt to do this.  Any screwup and you are having to replace the whole countertop or a section of it with all the issues of making it match.