Apology accepted! That took a lot of humility. Let it not be mention it again.
Your post mentions 2 distinct issues: Buzz (noise in the audio) and feedback.
Starting with the buzz issue, there are several aspects that I've dealt with in professional life when it comes to that. Most of the situations I've worked in are permanent facilities; however some of the principles can be applied to portable installs.
1. Balanced signal paths. If possible, use DI/isolation boxes to convert any unbalanced signals to balanced. One advantage is that most DI boxes have a "ground lift" switch that can make a difference (which you mentioned). Also, floating the shield at the source can help too. I used to have a set of directional cables with the shield floating at one end to reduce ground loops.
2. Power conditioning: There are power strips with extensive filtering built-in. Those can be a real help keeping stuff out of your audio. Most likely the light dimmers are on the same circuit as your audio gear and that can wreak havoc on audio systems. There are also transformers that are basically 1:1 and don't alter the voltage but act as a filter when plugged inline.
3. Instrument shielding: In the case of guitars, you'll want to shield the cavities and back/scratch plates.
4. Wireless transmission: Sometimes, eliminating a cable with a wireless system can help with buzzing issues. Bear in mind that depending upon the technology used and the frequencies employed, a wireless system can also add noise. If you have noisy wireless inputs, try replacing with a cable to see if there is any difference and work from there.
5. Ground lifting: This one is tricky. Removing the ground connection from the entire system can be dangerous in case of something going bad (like a power supply). But maybe you can choose your ground instead. In the studio, we disconnected the "Edison" power ground connection on all the equipment and instead ran a piece of #10 stranded to the chassis of each piece of gear, and ran those wires (soldered them onto) to a "000" length of welding cable that I personally connected to the street side of the water main in the building foundation. You may be able to find a good ground connection in the club and run a thick cable to that and then float the grounds on each piece of gear. Those ground lift plugs have a tab to connect a wire to, and that wire can go to your "star ground" that you attached to a water pipe. For the console and recorders you can try a computer type UPS and see what that does.
One of the problems is that when you're setting up for a show you don't have time to waste on troubleshooting. But, if time permits, connect the inputs to the console 1 x 1 and observe the results. There might be one thing that is causing the system wide issue.
Some of your gear plugs into the wall. Other things have a wall wort. Those things that run on DC from the wall wort don't have the same ground reference. So, it's possible to have micro current flow on the ground paths. It gets complicated but those isolation techniques can help break those loops.
) dealing with ground loops.
