well, you shouldn't have to throw it away. You have run Etrecheck which is helpful. I don't know what you found, but Etrecheck can help point the way to stuff that might be causing you problems. You also might want to run Activity Monitor (found in Utilities) which can help pinpoint which applications/processes are eating up cpu time and ram and then stop them while they are happening, at least for today. Also, under system preferences, then Users/Groups, check login items. If there is anything starting up when you login to your MacBook Pro and you don't want it to, that's where to stop it, or remove it entirely. You could also check your home folder. Go to your main hard drive, then under Users, then go to "yourname" which looks like a house. Click on that. Under Finder, click on Go, I think it's the 4th column from the top. Then go to folder, and type in Library. This is kinda different from the System Library. in the bunch of folders that open up, go to Application Support and Preferences. This is where some of the Applications put stuff needed to run them, such as PPPmonitord, either that or Preferences. You'd need to delete that application, then go back into your User Library, using the steps I mentioned, and look for anything that stars with "PPPMonitord". If not needed, click and drag it to the trash, and check the Preferences folder. Do the same there. Now empty the trash and restart your MacBook Pro. If you have a lot of launch deamons, that's probably where they have built a nest or moved into a cave ( I don't know where exactly they like to live, cave, nest, or shack, but they are there for sure) . As for #1, the part about the firewire and ethernet ports getting hot. That may be a hardware problem, which means you may need to take it in to your local Apple Authorized Service Place, have them take a look and see what they say. I don't know if Apple has said officially that the the MacBook Pro 2012 is vintage/obsolete, but it may very well be. So that would mean that your local Apple store will not touch it, or have parts for it anymore, so they might not be able to help you. I'd also suggest resetting the smc/pmu and zapping the pram on it. How to do that will depend on if it has a removeable battery or not. Check out Apple's support page on Notebooks and resetting smc/pmu and zapping pram. It should tell you how to do it, no matter which model/year of MacBook Pro , and what it's supposed to do.
hope this helps you a bit
John B