I assume WP5.1 only works in a DOS environment, including Win9x and the DOS emulation of NT. If whoever owned WP at the time produced ported it, they probably ported it to Mac.
2. Did Bill Gates invent DOS? Is there a DOS that is non-Gates? If so, where can I get it, or download it? I hear that some LINUX is difficult. Is there a simple LINUX that is basic to operate and uses a small amount of disk space? Or, is there some other simple OS that will work with WP5.1?
Bill Gates did not create DOS, he purchased an implementation of it. You can find DRDOS and PCDOS if you look hard enough, but I think it would be very foolish to use an ancient, unsupported operating system for any semi-serious work. FreeDOS is newer, and may be what you're looking for. If you want to use DOS on Linux (nb the capitalization) you will need to use a DOS emulator like dosemu in conjunction with some version of DOS.