It's Getting Worse

So this electricity thing. Of course the house would start falling apart right after a big, elective upgrade. Here's what's happened so far--

• A day or two into the new patio build, the landscaping company accidentally cut a wire to our house. We had it checked out and were told it was just an old copper phone line, so we dismissed everything that followed as unrelated.
• About three weeks ago, a couple days after the phone line was cut, we started noticing that the lights beginning to flicker pretty dramatically.
• After about a week of making sure we weren't crazy or needed our eyes checked, we tentatively linked the flickering to using the microwave, but we still weren't certain. The office was affected, with the computer plugged into a powerstrip attached to one of the sockets being forcibly shut down. No circuits in the basement were tripping but a GFCI was. 
• Then the same thing happened, but the light didn't start working again when we reset the GFCI. That brings us to where we were during my last post about the issue.
• D did some reading and consulting and decided that the microwave was the likely (and also cheapest, though microwaves aren't exactly "cheap" culprit). So we ordered one.
• He also solved the problem with the light in the office with some tinkering that required only minor electrical work and therefore was permissible according to my general sense of things you should and should not attempt with no professional training:
• Apparently, you can test the switch when the electricity is off, to see if it still can conduct power. This switch, apparently, failed the continuity test (see the 1? that means no continuity, even though it was switched on):

• A new switch that we just happened to have in the garage passed the continuity test with those 000s when the switch is in the "on" position.
  

• So D swapped out the switches and suddenly the light worked. Go us! And with a new microwave on the way, we figured we were on our way to solving this.
• Then the microwave arrived. We took a quick peek behind the wall before they put the new one in, just to make sure there wasn't any kind of visible issue there that could have caused the microwave to break. Nope, all good:

• So we have this new microwave (we selected that microwave like we select pretty much everything these days, by seeing what the Wirecutter recommends and buying it, because we're lazy. We decided to final switch from black (to match our oven and dishwasher) to stainless steel (to match our fridge) in the hopes that eventually we will end up with everything being stainless.

• And guess what. The new microwave solved literally nothing. Not only didn't it fix the existing flickers, but they seem like they are getting worse. Now we can consistently predict that the office will short whenever we start the dryer and even sometimes when the heat clicks on.The garbage disposal even significantly dims the lights.

An electrician came out to take a look and he checked the circuit box, tightened some things, and confirmed that all of these things are on separate circuits. Other than that, he had truly no idea what to do. He didn't charge for the visit, at least.

Then today, turning on the dryer even made the living room lights turn off, then on, then off, then on again. This problem seems to be spreading and we have no more leads and an unnecessary microwave (we actually miss our old one, but oh well).

Any ideas? Besides trying a new electrician.