Sometimes shifting into Park the P doesn't light up without jiggling the J gate, what happens if I were to leave it in Park without the light on and shut off the car, so the J gate electronic switch hasn't recognized the transmission is in the park position?

Would this effect the brake-to-shift safety mechanism? Alarm arming? Restating the car? Anything?

 

Biggest thing I would worry about is that if you are parked on a hill that it is not actually fully engaged in park. This can be due to the little "microswitch" that the lever closes when shift lever is placed in park, the linear switch itself, or a stretched cable not allowing the trans position relative to linear switch and lever position to be properly calibrated. I would find a hill, place the car in park, and make sure everything is copacetic before something bad happens. This is assuming it is an XJR, XJ8s are different. I would imagine transmission fault codes are right around the corner for you

edit: make sure you are in the car when you do the hill park test

 

Never leave the car in park on a hill without the park brake being engaged. It is a tiny pawl that is engaged and with the weight of the car on it, it won't be long before it breaks and then you are stuffed.

 

Sorry I mean anything but flat

 

Yes, I'm too self educated on car stuff to just Park on a steep incline, throw it in Park and abruptly release the break pedal, but I've seen dozens of other drivers do that...

If I jiggle the shifter it eventually engages to park, I'm thinking maybe just a stretched cable or dirty switches (so no codes likely from either possibility) but I'm wondering what happens if it gets to the point that I can't jiggle it into the electronic Park but the transmission cable is still in Park position.

 

It's N/A vdp (xj8 transmission, not the s/c xjr tranny)

 

It could just be a dirty or "sticky" switch, worth a look. A stretched cable will give you more problems as stretch progresses, like shifting problems, tranny temp faults, wrong gear selected messages, trans faults, or CEL.... on the R cars at least. Fix a few a year with X308/X100 R's at this age.
As for specifics to what will happen post "jiggle failure" I don't know