Pick an easy one, why don't you!
Stop me if you know this one .... You can pulse guide (using ASCOM/EQMOD) or use an ST-4 port on (presumably) your guide cam. Both will use PHD to do the guiding calculations.
AFAIK, the serial cable (adding a USB-to-RS232 convertor just adds USB to the computer end - it doesn't change the serial end) allows you to plug into the handset and use PC Direct mode or just to send GoTo coordinates (e.g. from Stellarium) in normal handset mode.
In PC Direct mode you
should be able to pulse guide, theoretically, but I've not tried it. However, most people recommend not trusting PC Direct mode and using an EQMOD interface (e.g.
Shoestring USB2-EQ6) to bypass the handset altogether. It's proven and may be safer for your mount.
You can't use the USB/Serial cable to bypass the handset, even if you could wire it up, as the voltages are (well, maybe) different.
HTH.