Hi Barry,
First I want to stress that I'm in no way an expert on motor control, I only know what I've learned from my own experiments. There will certainly be things which I either don't know, is missing or even get wrong.

1/. Can a locked antiphase arrangement be configured on a PIC18F2431 using the PCPWM Module? If not, can it be done via other hardware PWM modules within this PIC?
Yes but as far as I know not the way you have it wired now. You need one PWM generator in complementary mode and wire it so that PWM0 drives the upper right and lower left switch while PWM1 drives the upper left and lower right switch.

2/. I have read in other posts on the forum that your incPID routine will work with locked antiphase configuration. Please confirm this is the case.
If you've read that you've probably also read that the incPID routine does not know, nor does it care, what it is controlling or what you do with the value it puts out - all it is doing is crunching numbers. It's up to you as the user of it to take its output and apply it accordingly. Be it locked antiphase PWM, basic "unipolar" PWM, sign magnitude PWM, a +/-10V DAC or whatever, it doesn't matter.

3/. Will regenerative braking become a major issue to handle for a 180Vdc 1500W treadmill motor with a rectified mains supplied power supply?
I don't think regenerative braking per se will be a problem but I think you need to make sure you don't allow "instant" reversals or stops - like you explained earlier. Never allow the setpoint value to change from one value to another "instantly", always apply ramping and I think you should be fine. Especially so since you don't really have an active current limiting scheme.

If you rectify your 240V mains you'll end up with almost twice the rated voltage of the motor....

I guess the only way to find out is try it. Be careful though!

/Henrik.