A lot of this depends on your local building codes, so YMMV.
Low voltage wiring is generally not an issue in wall, and most commercial cabling is compliant.
Sanus offers incentives to installers, so there is a significant markup on those products. The installer is likely steering you away from the Monoprice product for that reason. He can't make any money selling you something you already own. There are many satisfied Monoprice owners here.
Not sure of your configuration. If it is a newer TV feeding into a newer AV receiver, the system likely has the Audio Return Channel (ARC) built into the HDMI cable. In that case, there is no need for an optical cable. Everything can and should be done with power.
Here is what I think you actually need:
1. Level and mount the monoprice mounts on the wall (45 minutes labor)
2. run a power cable (romex) from the lower outlet into a new power outlet behind the TV ($10 parts, 1 hour labor)
3. install 2 low voltage cover plates and fish the HDMI cable. I would use the ones that look like brushes for convenience. ($15 parts, 45 minutes labor)
4. Mount TV and make connections (30 minutes labor)
Total is $25 in new parts from Home Depot, and about 3 hours labor. The labor estimate is generous. Every installer will charge a premium for the parts, so figure $50. Job should not cost more than $250/TV.
This is something you can do yourself or have a handy relative do for you. Not rocket science (although I am an aerospace engineer)