How can I uninstall a Windows Service even if I'm missing the .exe for it?

You should be able to do it using below command:

sc.exe delete <service name>

If you do not have sc on your system... check your resource disk.

How can I grep multiple patters?

Try:

grep -i "pattern1|pattern2|pattern3"

Listing files which contain a specific string

Try:

grep -li "mystring" *

l --> Outputs the name of files.
i --> Ignore Case.