Tessnet2 is multi threaded. It uses the engine the same way Tesseract.exe does. Tessdll uses another method (no thresholding).
Tessnet2 is under Apache 2 license (like tesseract), meaning you can use it like you want, included in commercial products. You can read full license info in source file.
Quick Tessnet2 usage
Download binary here, add a reference of the assembly Tessnet2.dll to your .NET project.
Download language data definition file here and put it in tessdata directory. Tessdata directory and your exe must be in the same directory.
Look at the Program.cs sample