>However, we discovered that using the Web Service as multi-threaded has some impact

I guess that speed impact also? but is safer, anyway I don't use yet the MTDLL only single-threaded.

>IISRESET /RESTART. This method is longer and shuts down the IIS services while it is being done.

IISRESET witouth the RESTART parameter works for me.

>Another approach would be to define the Web Service is a specific virtual directory, define it as Isolated and use the Unload option from MMC everytime we have to compile. By that, we only shut down the components from that directory.

Also in the Medium (grouped) I can see the Unload button activated, didn't tried.