MiniGrief

This is probably the easiest solution if you don't mind spending a little money to get something up with little effort. 
 
There is always the option to host it yourself, which is free.
 
To sum up what you have to do:
Send the client files to anyone wanting to play. The editor program doesn't need to be sent if they don't need it.
You will also need to modify the config.xml file inside the resources folder of the client so the IP matches your public IP. You can find this by typing "What is my IP" into Google.
Provided UPNP is working fine for you, port forwarding should not be needed. However, if it is, you can look up a guide for your router on how to port forward.
 
If all that has worked fine, everyone should be able to join fine. Keep in mind that any updates to client files need to be sent to the users.
 

The only way I know of doing this (without modifying the source) is to distribute your game with a .reg file to modify the section of the registry that stores the resolution selected. Though, this could be seen as a shady thing to do since a .reg file could do more than simply set the resolution of the game. I believe it also boots into 800x600 default as some people still use monitors at that resolution. Just a backwards compatibility thing.
 
You'll be better off just telling people that you recommend changing the resolution if their monitor supports it.

That XML file works fine.
 
You are probably missing one or more of "event2responses.png", "event3responses.png" and "event4responses.png" from your GUI folder. I removed these files and it created the same problem.
 

If the GPU listed from the error in your first post is what you have, that will be why it will not run. That is a fairly old GPU and only has 256 MB of VRAM. The requirements on the documentation show that the editor requires 1 GB of VRAM.
This is likely the problem.
 
https://www.freemmorpgmaker.com/docs/en/Welcome.html#page_Requirements

For Z-Dimensions, add this:
<ZDimensionVisible>True</ZDimensionVisible> to your server's config.xml like this:

It looks like you haven't entered your PC's IP on the network properly into your port settings.
 
Open CMD and type ipconfig and then this should bring up some information on your network connection. You need to look for IPv4 Address in the information and this should be very easy to find provided you don't have more then one adapter installed. It will look like 192.168.0.17 but the 17 will likely be a different number. You need to change the Internal IP in the port settings to the number found in your CMD.
 
Another possible fix is enabling UPnP. This will deal with the port forwarding for you. Very few older routers support it though and it is usually enabled by default if the router does support it.
 
This is the only problem I can find with your setup, but it might not be all of the problem(s) as port forwarding can be fiddly sometimes.

https://drive.google.com/open?id=1ul9j72HzsE1HmyIj4Ye_Js-S5l80ssOs
 
They are unorganized, but it should hopefully be a start for you.
There's just so many that the usual method I use for splitting wouldn't work.

There is no real answer to this. The only true way to know is to actually have the amount of people you're expecting to play to actually play at the same time.
Personally, I believe that 100-200+ should be no problem if everything is perfect about your set up. This really does depend on the specs of the system it is running on and the speed/stability of the network though.
 
There will never be a truely perfect set up.

I think you can change them in the server's config.xml.
 
Edit: Just checked and yes. Here's an example of what you do if you're adding a tool called "Extra Tool"
<Slot4>Extra Tool</Slot4> Add that into the  <ToolTypes> area.

A pretty good guide on how to port forward is here: Guide
 
Port forwarding might not be the same for you as shown in the guide but it should hopefully give you a good idea on how to go about doing it.