place the speaker in such a way that the reflecting sound wave does not come right back to the speaker and cannot enhance the signal.
This is SBIR, which can be constructive if the reflections are in phase with the original wave, or destructive if the reflections are out of phase with the original signal.
why then all this business with REW testing to find the optimal speaker location
REW is a good tool to discover problems in your room (peaks, nulls, long decay times, distortion, etc), so you can address them with the right type of acoustic treatment. An untreated room's waterfall graph is going to be a train wreck but you can start by measuring a few different speaker locations to find the best location in that particular room. Then you begin attacking problem frequencies with acoustic treatment.
The goal is to minimize SBIR with your speaker placement, or use it to your advantage. In most residential rooms there is rarely enough space to do sufficient bass trapping behind the speakers so usually in a typical residential setup the best position for the speakers is centered on the short wall, directly against the wall. This will help ensure the rear reflections are as in phase with the original wave as possible, resulting in a boosted low end, which you can then compensate for by reducing the bass trim on your monitors. Obviously if the reflections are out of phase, you'll get a dip in frequency response, which cannot be fixed with room correction software or by boosting certain frequencies because the out of phase reflections will also be boosted. The same goes for nulls created by room modes but that's a different topic.
Crude math illustration might help: problem frequency original wave amplitude + reflection amplitude = resulting amplitude hitting your ears
In phase: 5 + 5 = 10 (will show up as a peak in REW)
Out of phase: 5 + (-5) = 0 (will show up as a dip in REW)
Out of phase but boosted to compensate: 10 + (-10) = 0 (roughly the same dip)
What exactly happens when a note C3 (130HZ) tone leaves the speaker? It gets reflected on the walls and causes resonance of the original 130Hz sound which gets louder and louder? It increases vibration of my string? Or does 130Hz excite standig room waves that finally by itself make the "loudness" of my C3 note go way up? Appreciate if someone with patience could explain that in dummie fashion. I searched the internet and there are lots of articles but I still dont really understand what exactly is going on, who does what to whom in what fashion. Thanks a lot for bearing with me.
Just do some research on room modes, youtube is helpful. Here is a good article, read all 4 pages. For some reason it displays weird in my browser so make a PDF if you need to.
https://arqen.com/acoustics-101/room-setup-speaker-placement/
Forget about the specific feedback issue on the guitar in this instance until you get your room's issues somewhat under control. If you're going to DIY some panels with building insulation don't bother making anything under 6" deep because shallow velocity-based absorbers can only absorb high frequencies due to the longer wavelength of low frequencies, so you'll just end up making your room sound dead and won't address the low end issues which require much deeper absorbers, especially if they're velocity-based i.e. building insulation.