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Is 7+ Taskbar Tweaker still running? Could it be that it's killed, maybe by an antivirus?

The reason for the incompatibility is that both 7+ Taskbar Tweaker and StartIsBack hook the same function, and one of the hooks takes over the other. I tried to revert the relevant portion of the changes from v5.11.3, please try beta v5.14.1.2 which I just released and let me know whether it solves the incompatibility.

You can configure middle click to minimize the window. 

There was a problem calling GetProcAddress. Generally that shouldn't happen, but I remember seeing this issue when 7+ Taskbar Tweaker was running in compatibility mode. Please make sure that all compatibility options are off. If no compatibility options are enabled, my next guess would be a conflict with another program, such as another customization tool or your antivirus.

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Yes, it's called win7appid as it was created for Windows 7 back when there was no Windows 10, but it's relevant for all Windows versions not older than Win7.

That's not possible with 7+ Taskbar Tweaker. As a workaround, you can try creating a custom shortcut with the win7appid tool and configure that AppID in the tweaker.

Sounds like the disable_topmost advanced option gets disabled when mIRC is maximized. I might look at it one day, but it sounds rather specific so it's not a priority.