GL.iNet’s own video said to SSH with Ethernet. I only use the router for WiFi and my laptop doesn't have an Ethernet port. Can I install the Ctrld CLI via WiFi? Can I SSH via WiFi? If not, can I use a USB-C to Ethernet adapter to SSH into the GL.iNet router?
In the event I do a factory reset of the router for the purpose of selling it, will the Ctrld CLI be removed from the router? If not, how do I permanently remove it?
You can ssh into the router via WIFI or using a USB Ethernet adapter. I use both methods all the time. If you are still using the default IP address, use:
ssh root@192.168.8.1
with the same password you setup in the GUI
Anything you add, such as the Control-D software, will be deleted any time you reset or upgrade the firmware. You should keep copies of everything, and document your steps, if you ever need to recover it.
I use the Control-D ctrld program on Ubuntu and Debian systems, but have never tried to put it on a GL iNet router. They have their own way of doing DNS, so you will probably have to figure out how to disable it first, if you want to use the Control-D DNS software. It probably will not be easy or clean.
Is it possible to use my iPhone or Google Pixel to SSH via WiFi and install Ctrld? If so, how? Can you please provide instructions?
There are apps for both phones for you to SSH into a remote device. Just google “Phone in question SSH client” and you will be presented with different options to meet your needs for each brand you listend
I use termius on android to connect to the router.
Note that when you factory reset the router, the ssh password defaults also back to routers factory password.
Depending on GL repo you may try to search if there is a ctrl daemon plugin, if there is not proceed with the normal Controls wiki for installment on linux through ssh.
You may need some dependency and maybe not these might be obtainable from the plugins since not all things and dependencies are build on arm as cpu, so you want to keep as close on the plugins section for these things 
^ there is also a chance that the normal controld script fails because the package manager is expected to be apt but OpenWrt uses opkg or apk, just read carefully the error and install it via the plugins system.
I don't understand. Are you able to tell me which package to download, either iOS or Google Pixel, and the specific steps to add the CLI to my GL.iNet router?
Basically I ment to say with this post you must trail and error in order if you miss dependencies or not, it will be very obvious when the install fails what you are missing.
I'm not talking about a ssh client, but purely about setup script for control cli what i linked in the wiki page, and it is linux.