Welcome to JustAnswer and thank you for using us. I have experience with Ubuntu and I should be able to assist. I will start with two questions and further ones as we progress
1. What are the contents of this file /etc/X11/default-display-manager ?
2. Just so we can rule out some other problems, since you have already looked at some blogs have you already tried this one https://community.linuxmint.com/tutorial/view/842 ?
Ok thank for the update. I am a little held up right now I'll opt out for now just to allow other experts to contribute. I'll get back to it later today.
Hello. Can't do secure remote assistance if the box won't even boot. It isn't a good idea if you "don't know what [you're] doing" (YOUR OWN WORDS) to mess around with various add-ons at the command-line level. Reinstall FROM SCRATCH and get it to boot COMPLETELY, ALL BY ITSELF, before undertaking any modifications. Remember: Linux has degraded DRAMATICALLY over the last ten years, and you are working on a hybrid that every would-be "expert" has contributed to, resulting in a hodgepodge that now exhibits the level of professionalism formerly chargeable only to Microsoft Windows.
If you are getting a login prompt, then you specified the wrong option during installation. I hate to tell you to go through all that jazz again, but please go through all that jazz again and pay EXTREMELY CLOSE ATTENTION. You will be asked whether you want logins to default to TERMINAL MODE or X-WINDOWS/VISUAL MODE (in which case a program called XDM, the X-WINDOWS DISPLAY MANAGER, will pop up to collect your username and password. This is different from the login "prompt" you describe because it's within a dialogue box with "OK" and "Cancel" buttons.
I can't give you remote instructions for how to configure XDM to run by default. That is driven through the /etc/lines or /etc/inittab file that determines, for each available input connection, what process to run, i.e., whether a GETTY (which collects login and password ***** dull, command-line-looking prompts) or whether an XDM (which will post a dialogue box to collect your credentials). Such instructions would only cause confusion. If you redo that part of the installation, YOU WILL BE PROMPTED to choose console-mode login or graphical-mode login for the default session style.
I agree, the approach sounds tragic, but it's the only way to assure that all the dependencies (far-reaching "tentacles," if you will) are taken care of. Linux is NOT what it used to be. It is a profoundly disorganized mess. If you wanted to run a real UNIX system, robustly built for professionals, the home user's only choice nowadays is Apple.
It sounds to me like you're happy just trying whatever random measures to get the problem fixed. Good luck!