SharpDevelop WinForms Designer
It is still your best chance to get an open source implementation. You'd better define what is "unusable".
JetBrains Rider's WinForms designer is probably built upon this code base, but they might have upgraded a lot but won't release the code.
An open source project like that won't die and you can see people work in their forks https://github.com/icsharpcode/SharpDevelop/network . If a new maintainer steps in, such can be revived.
Keep in mind that WinForms is rather complex a framework, so SharpDevelop's designer might work for many cases but not all.
Microsoft is currently working on a new designer (primarily for WinForms on .NET Core, but later might also support .NET Framework), so Visual Studio's designer is still the best in the market.
Mono
Pay attention to your words like "due to Microsoft's business tactics".
Mono WinForms could not continue as Linux is not Windows, and technical barriers block its development. There isn't an ecosystem on Linux either as you cannot port Windows dependent controls (open source or commercial) to Linux.
Thus, there was never a visual designer for WinForms on Mono side, but a GTK# visual designer. The Mono guys moved their focus to other areas and successfully discovered the new landscape of mobile and built new stuffs there.
So you shouldn't waste your time digging MonoDevelop code base.