When MS set up the mini WAN port it commandeered the NIC and turned it into a router.
No, it takes at least two NIC's to make a router. One NIC cannot be a router. A
"router" directs traffic between two (or more) dissimilar networks (like the internet and your home LAN). You can turn a computer into a router, but it needs at least two NIC's inside. A NIC can be either wired or wireless.
I know even when my HP Laptop was up and running every so often I would get this message "there is another device using the same IP address. I got this once when I set up this computer but the other day when I was trying to install the router I received the same message. In other words there was a conflict of addresses.
How can this arise?
This is a configuration problem. If you use just one router, and it uses DHCP to issue IP addresses to the computers in your home LAN, I don't think you will ever have this problem. That you see this error is telling you that you have a configuration problem. You may or may not be using DHCP... this is set in your ISP's router (the "box")... and you've not looked into that. This is your first step to solve your problem. You must know how the ISP router is configured so that you can configure your LAN to match it.
There is nothing to indicate that any hardware in your network is defective... not the ISP router (the "box"), not any NIC's in any computer, not your switch, and not your new MicroTik. But adding new devices only complicates your problem until you figure out how to properly configure your network. I don't think you need to buy yet another WiFi device to make your wife's laptop work. The MicroTik should work just fine. But again, any device needs the settings of the local network so that it can be configured properly. The "Quick Start Guide" for the MicroTik says it is "pre-configured" and should work as soon as you plug it in, but it seems that is not true in your case, so you may need to configure it manually.
You may can get enough information from your Windows 7 network settings, since it's the only thing that works... but I'm not sure. (We have failed so far.) You may still need to look at your ISP router settings too. I strongly recommend that you get someone locally who knows how to setup a home network to come over and assist you, and get that person to setup your Linux Mint and your MicroTik while they are there... a complete job. You need someone knowledgeable who can physically see your equipment and can step through all the
many different pages of settings in your ISP router, if needed. If your router is using Static IP Addressing instead of DHCP, then you need to make a full list of the static IP addresses that each device on your network uses. You set these IP addresses manually yourself on each device, so you have to be sure that you do not create duplicates. If you use this method, you will need to set up the IP address on the new laptop that you will buy for your wife.
I believe I have come to an understanding why I cannot access the net through Linux Mint.
The same computer and same NIC works with Windows... and Mint doesn't work. So Mint is not configured properly to match your LAN. From the previous Windows settings you showed us, and since standard Ethernet over DHCP does not work, I am pretty certain that you need to "Add Network Connection" with your Network Manager and enter the proper settings. We suspect these to be PPPoE (and CHAP authentication), but you have not successfully done this. There may be more configuration information needed that we have not discovered. Mint will work, I have no doubt in my mind at all. But you must set it up properly to match your local network.
You cannot identify what your ISP router brand/model is (no markings, you said earlier)... so it would be very risky for us to try to instruct you on how to log into it and investigate its settings. If you change something in there then you may not have any internet at all.... that would be bad. Your ISP may have provided documentation to you that says all we need to know about the router's settings, but we haven't heard that from you either. Mint only needs to be configured properly, and it will talk to your router and to the internet.