Electronic Lead Screw

I have a 7x14 manual lathe that I'm converting to CNC and want to use LinuxCNC. I have a couple questions regarding threading.
There are numerous Electronic Lead Screw (ELS) projects done using a microcontroller reading a spindle encoder and driving a stepper on the lead screw. These are interesting becuase they are synced even if the spindle is off. So just like in a lathe with change gears, if you turn the spindle by hand, the lead screw follows. Here are my questions:

Can LinuxCNC keep the lead screw synced with the spindle even if the spindle is turned by hand or does it only drive the lead screw when a program is running?

I have a 360 pulse per revolution(PPR) encoder coupled to the spindle. Becuase it's a quadrature encoder, I think this generates 1440 PPR. At 500 RPM, this would be 12kHz. Is it realstic to expect to read this into a parallel port on an i7 PC?

The ELS projects only use a quadrature input. I looks like most LinuxCNC spindle encoders have an 1 PPR index pulse also, Is this a requirement to thread on a LinuxCNC lathe?

Thank you, Jerry

Syncing off a single PPR was added to LinuxCNC to support people migrating from Mach3 (and before that TurboCNC) which only supported this. LinuxCNC natively supports syncing with proper quadrature encoder signals.

I am guessing that issuing a G33 and then manual turning the spindle will result in correct syncronised motion. I guess you are thinking about cutting large threads beyond the torque limits of the 7x14 spindle drive.

edit;
you can expect > 25kHz from a parallel port, so it should be fine at 500rpm. Since the spindle speed feedback will probably also be driven by this signal, once you crank up the RPM, and exceed the reliable read frequency of the LPT, you will probably get an incorrect RPM displayed.
Mark