`sudo apt update` returns unverified and unsigned signatures due to unavailable public keys? This stemmed from wanting to reinstall wine

Question

I have a hard drive which recently died, instead of paying for a new copy of Windows I went with an Ubuntu distribution.

I've been trying to run a couple different .exe files through WINE, which hasn't been working. I figured I could try a fresh reinstall of wine and ran through these steps as proposed by an article on PCsuggest posted in 2018:

sudo apt-get purge wine wine-*     
sudo apt-get autoremove --purge     
rm -rf ~/.wine/     
rm -rf ~/.local/share/applications/wine*

I then ran through the steps laid out to install Wine from the WineHQ site for Ubuntu 18.04. Which went well up until the

sudo apt-add-repository 'deb https://dl.winehq.org/wine-builds/ubuntu/ bionic main' 

line, which didn't seem to work. Nevertheless, I'm pig-headed enough to keep moving forwards, so then I tried sudo apt update, which definitely didn't work. That command returned this:

Ign:1 https://dl.google.com/linux/chrome/deb stable InRelease  
Hit:2 https://download.mono-project.com/repo/ubuntu stable-bionic InRelease                                                                          
Hit:3 https://dl.google.com/linux/chrome/deb stable Release                                                                                           
Hit:5 https://repo.steampowered.com/steam precise InRelease                                                                                           
Hit:6 https://dl.winehq.org/wine-builds/ubuntu bionic InRelease                                                      
Hit:7 https://us.archive.ubuntu.com/ubuntu bionic InRelease                                          
Hit:8 https://us.archive.ubuntu.com/ubuntu bionic-security InRelease                                 
Hit:9 https://us.archive.ubuntu.com/ubuntu bionic-updates InRelease                                
Ign:12 https://ppa.launchpad.net/cairo-dock-team/ppa/ubuntu bionic InRelease                       
Hit:13 https://ppa.launchpad.net/diesch/testing/ubuntu bionic InRelease        
Ign:14 https://ppa.launchpad.net/falk-t-j/qtsixa/ubuntu bionic InRelease        
Get:4 https://ppa.launchpad.net/otto-kesselgulasch/gimp/ubuntu bionic InRelease [15.4 kB]  
Err:4 https://ppa.launchpad.net/otto-kesselgulasch/gimp/ubuntu bionic InRelease  
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3BDAAC08614C4B38  
Get:11 https://ppa.launchpad.net/libreoffice/ppa/ubuntu bionic InRelease [20.7 kB]  
Err:11 https://ppa.launchpad.net/libreoffice/ppa/ubuntu bionic InRelease          
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 83FBA1751378B444  
Err:15 https://ppa.launchpad.net/cairo-dock-team/ppa/ubuntu bionic Release        
  404  Not Found [IP: 91.189.95.83 80]  
Err:16 https://ppa.launchpad.net/falk-t-j/qtsixa/ubuntu bionic Release  
  404  Not Found [IP: 91.189.95.83 80]  
Reading package lists... Done  
W: GPG error: http://ppa.launchpad.net/otto-kesselgulasch/gimp/ubuntu bionic InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3BDAAC08614C4B38  
E: The repository 'http://ppa.launchpad.net/otto-kesselgulasch/gimp/ubuntu bionic InRelease' is not signed.  
N: Updating from such a repository can't be done securely, and is therefore disabled by default.  
N: See apt-secure(8) manpage for repository creation and user configuration details.  
W: GPG error: http://ppa.launchpad.net/libreoffice/ppa/ubuntu bionic  
InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 83FBA1751378B444  
E: The repository 'http://ppa.launchpad.net/libreoffice/ppa/ubuntu bionic InRelease' is not signed.  
N: Updating from such a repository can't be done securely, and is therefore disabled by default.  
N: See apt-secure(8) manpage for repository creation and user configuration details.  
E: The repository 'http://ppa.launchpad.net/cairo-dock-team/ppa/ubuntu bionic Release' does not have a Release file.  
N: Updating from such a repository can't be done securely, and is therefore disabled by default.  
N: See apt-secure(8) manpage for repository creation and user configuration details.  
E: The repository 'http://ppa.launchpad.net/falk-t-j/qtsixa/ubuntu bionic Release' does not have a Release file.  
N: Updating from such a repository can't be done securely, and is therefore disabled by default.  
N: See apt-secure(8) manpage for repository creation and user configuration details.  

I should mention, earlier the problem was a broken line in the sources.list file related to WINE. Which I fixed by realizing that (I think?) I had a sources.list file for Ubuntu 16.04, which is the version I installed initially onto my laptop from a flashdrive before updating to 18.04. So I moseyed on over to https://repogen.simplylinux.ch/ and downloaded a sources.list file for 18.04 and replaced the old one. I have both files backed up as text files on my desktop so if I need to put that 16.04 file back in, I think I can do that.

Any help would be appreciated.